[llvm] [CMake] Use shm_open, not clock_gettime, to detect librt (PR #120727)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-s390x-linux-multistage` running on `systemz-1` while building `llvm` at step 5 "ninja check 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/98/builds/1080

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 5 (ninja check 1) failure: stage 1 checked (failure)
******************** TEST 'libFuzzer-s390x-default-Linux :: fuzzer-timeout.test' FAILED ********************
Exit Code: 1

Command Output (stderr):
--
RUN: at line 1: /home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/stage1/./bin/clang    -Wthread-safety -Wthread-safety-reference -Wthread-safety-beta   --driver-mode=g++ -O2 -gline-tables-only -fsanitize=address,fuzzer -I/home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/llvm/compiler-rt/lib/fuzzer  /home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/llvm/compiler-rt/test/fuzzer/TimeoutTest.cpp -o /home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/stage1/runtimes/runtimes-bins/compiler-rt/test/fuzzer/S390XDefaultLinuxConfig/Output/fuzzer-timeout.test.tmp-TimeoutTest
+ /home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/stage1/./bin/clang -Wthread-safety -Wthread-safety-reference -Wthread-safety-beta --driver-mode=g++ -O2 -gline-tables-only -fsanitize=address,fuzzer -I/home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/llvm/compiler-rt/lib/fuzzer /home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/llvm/compiler-rt/test/fuzzer/TimeoutTest.cpp -o /home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/stage1/runtimes/runtimes-bins/compiler-rt/test/fuzzer/S390XDefaultLinuxConfig/Output/fuzzer-timeout.test.tmp-TimeoutTest
RUN: at line 2: /home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/stage1/./bin/clang    -Wthread-safety -Wthread-safety-reference -Wthread-safety-beta   --driver-mode=g++ -O2 -gline-tables-only -fsanitize=address,fuzzer -I/home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/llvm/compiler-rt/lib/fuzzer  /home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/llvm/compiler-rt/test/fuzzer/TimeoutEmptyTest.cpp -o /home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/stage1/runtimes/runtimes-bins/compiler-rt/test/fuzzer/S390XDefaultLinuxConfig/Output/fuzzer-timeout.test.tmp-TimeoutEmptyTest
+ /home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/stage1/./bin/clang -Wthread-safety -Wthread-safety-reference -Wthread-safety-beta --driver-mode=g++ -O2 -gline-tables-only -fsanitize=address,fuzzer -I/home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/llvm/compiler-rt/lib/fuzzer /home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/llvm/compiler-rt/test/fuzzer/TimeoutEmptyTest.cpp -o /home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/stage1/runtimes/runtimes-bins/compiler-rt/test/fuzzer/S390XDefaultLinuxConfig/Output/fuzzer-timeout.test.tmp-TimeoutEmptyTest
RUN: at line 3: not  /home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/stage1/runtimes/runtimes-bins/compiler-rt/test/fuzzer/S390XDefaultLinuxConfig/Output/fuzzer-timeout.test.tmp-TimeoutTest -timeout=1 2>&1 | FileCheck /home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/llvm/compiler-rt/test/fuzzer/fuzzer-timeout.test --check-prefix=TimeoutTest
+ not /home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/stage1/runtimes/runtimes-bins/compiler-rt/test/fuzzer/S390XDefaultLinuxConfig/Output/fuzzer-timeout.test.tmp-TimeoutTest -timeout=1
+ FileCheck /home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/llvm/compiler-rt/test/fuzzer/fuzzer-timeout.test --check-prefix=TimeoutTest
RUN: at line 12: not  /home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/stage1/runtimes/runtimes-bins/compiler-rt/test/fuzzer/S390XDefaultLinuxConfig/Output/fuzzer-timeout.test.tmp-TimeoutTest -timeout=1 /home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/llvm/compiler-rt/test/fuzzer/hi.txt 2>&1 | FileCheck /home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/llvm/compiler-rt/test/fuzzer/fuzzer-timeout.test --check-prefix=SingleInputTimeoutTest
+ not /home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/stage1/runtimes/runtimes-bins/compiler-rt/test/fuzzer/S390XDefaultLinuxConfig/Output/fuzzer-timeout.test.tmp-TimeoutTest -timeout=1 /home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/llvm/compiler-rt/test/fuzzer/hi.txt
+ FileCheck /home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/llvm/compiler-rt/test/fuzzer/fuzzer-timeout.test --check-prefix=SingleInputTimeoutTest
RUN: at line 16: /home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/stage1/runtimes/runtimes-bins/compiler-rt/test/fuzzer/S390XDefaultLinuxConfig/Output/fuzzer-timeout.test.tmp-TimeoutTest -timeout=1 -timeout_exitcode=0
+ /home/uweigand/sandbox/buildbot/clang-s390x-linux-multistage/stage1/runtimes/runtimes-bins/compiler-rt/test/fuzzer/S390XDefaultLinuxConfig/Output/fuzzer-timeout.test.tmp-TimeoutTest -timeout=1 -timeout_exitcode=0
INFO: Running with entropic power schedule (0xFF, 100).
INFO: Seed: 2340201058
INFO: Loaded 1 modules   (13 inline 8-bit counters): 13 [0x2aa3e2d0e78, 0x2aa3e2d0e85), 
INFO: Loaded 1 PC tables (13 PCs): 13 [0x2aa3e2d0e88,0x2aa3e2d0f58), 
INFO: -max_len is not provided; libFuzzer will not generate inputs larger than 4096 bytes
INFO: A corpus is not provided, starting from an empty corpus
#2	INITED cov: 2 ft: 2 corp: 1/1b exec/s: 0 rss: 31Mb
#406	NEW    cov: 3 ft: 3 corp: 2/4b lim: 6 exec/s: 0 rss: 32Mb L: 3/3 MS: 4 CopyPart-ShuffleBytes-ChangeBinInt-InsertByte-
#418	REDUCE cov: 3 ft: 3 corp: 2/3b lim: 6 exec/s: 0 rss: 32Mb L: 2/2 MS: 2 CMP-EraseBytes- DE: "\377\377"-
#460	REDUCE cov: 4 ft: 4 corp: 3/4b lim: 6 exec/s: 0 rss: 32Mb L: 1/2 MS: 2 CopyPart-EraseBytes-
#48116	REDUCE cov: 5 ft: 5 corp: 4/7b lim: 477 exec/s: 0 rss: 35Mb L: 3/3 MS: 1 InsertByte-
#48197	NEW    cov: 6 ft: 6 corp: 5/9b lim: 477 exec/s: 0 rss: 35Mb L: 2/3 MS: 1 EraseBytes-
ALARM: working on the last Unit for 1 seconds
       and the timeout value is 1 (use -timeout=N to change)
MS: 2 InsertByte-CopyPart-; base unit: 94dd9e08c129c785f7f256e82fbe0a30e6d1ae40
0x48,0x69,0x21,0x48,0x69,
Hi!Hi
artifact_prefix='./'; Test unit written to ./timeout-ce1f2406f5850820b1e93fb6e1f47fa98e38f842
Base64: SGkhSGk=
==3867716== ERROR: libFuzzer: timeout after 1 seconds
AddressSanitizer:DEADLYSIGNAL
=================================================================
AddressSanitizer:DEADLYSIGNAL
=================================================================
AddressSanitizer: CHECK failed: asan_report.cpp:199 "((current_error_.kind)) == ((kErrorKindInvalid))" (0x1, 0x0) (tid=3867716)
    <empty stack>

MS: 2 InsertByte-CopyPart-; base unit: 94dd9e08c129c785f7f256e82fbe0a30e6d1ae40
0x48,0x69,0x21,0x48,0x69,
Hi!Hi
artifact_prefix='./'; Test unit written to ./crash-ce1f2406f5850820b1e93fb6e1f47fa98e38f842
Base64: SGkhSGk=

...

```

</details>

https://github.com/llvm/llvm-project/pull/120727