[compiler-rt] Fix prctl to handle PR_GET_PDEATHSIG. (PR #101749)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-clang-x86_64-gcc-ubuntu` running on `sie-linux-worker3` while building `compiler-rt` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/174/builds/3002

Here is the relevant piece of the build log for the reference:
```
Step 6 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'SanitizerCommon-asan-x86_64-Linux :: Linux/prctl.cpp' FAILED ********************
Exit Code: 1

Command Output (stderr):
--
RUN: at line 1: /home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/build/./bin/clang  --driver-mode=g++ -gline-tables-only -fsanitize=address  -m64 -funwind-tables  -I/home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/compiler-rt/test -ldl /home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/compiler-rt/test/sanitizer_common/TestCases/Linux/prctl.cpp -o /home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/build/runtimes/runtimes-bins/compiler-rt/test/sanitizer_common/asan-x86_64-Linux/Linux/Output/prctl.cpp.tmp &&  /home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/build/runtimes/runtimes-bins/compiler-rt/test/sanitizer_common/asan-x86_64-Linux/Linux/Output/prctl.cpp.tmp /home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/compiler-rt/test/sanitizer_common/TestCases/Linux
+ /home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/build/./bin/clang --driver-mode=g++ -gline-tables-only -fsanitize=address -m64 -funwind-tables -I/home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/compiler-rt/test -ldl /home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/compiler-rt/test/sanitizer_common/TestCases/Linux/prctl.cpp -o /home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/build/runtimes/runtimes-bins/compiler-rt/test/sanitizer_common/asan-x86_64-Linux/Linux/Output/prctl.cpp.tmp
+ /home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/build/runtimes/runtimes-bins/compiler-rt/test/sanitizer_common/asan-x86_64-Linux/Linux/Output/prctl.cpp.tmp /home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/compiler-rt/test/sanitizer_common/TestCases/Linux
=================================================================
==2183529==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x7f28a4d00044 at pc 0x562f23ebeaf5 bp 0x7ffdcf0e30d0 sp 0x7ffdcf0e2880
WRITE of size 8 at 0x7f28a4d00044 thread T0
    #0 0x562f23ebeaf4 in prctl /home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/compiler-rt/lib/asan/../sanitizer_common/sanitizer_common_interceptors.inc
    #1 0x562f23f7ec57 in main /home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/compiler-rt/test/sanitizer_common/TestCases/Linux/prctl.cpp:45:9
    #2 0x7f28a6ffb082 in __libc_start_main /build/glibc-LcI20x/glibc-2.31/csu/../csu/libc-start.c:308:16
    #3 0x562f23e9d34d in _start (/home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/build/runtimes/runtimes-bins/compiler-rt/test/sanitizer_common/asan-x86_64-Linux/Linux/Output/prctl.cpp.tmp+0x2c34d)

Address 0x7f28a4d00044 is located in stack of thread T0 at offset 68 in frame
    #0 0x562f23f7e90f in main /home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/compiler-rt/test/sanitizer_common/TestCases/Linux/prctl.cpp:27

  This frame has 5 object(s):
    [32, 40) 'cookie' (line 36)
    [64, 68) 'signum' (line 44) <== Memory access at offset 68 overflows this variable
    [80, 161) 'invname' (line 52)
    [208, 214) 'vlname' (line 52)
    [240, 256) 'name' (line 74)
HINT: this may be a false positive if your program uses some custom stack unwind mechanism, swapcontext or vfork
      (longjmp and C++ exceptions *are* supported)
SUMMARY: AddressSanitizer: stack-buffer-overflow /home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/compiler-rt/test/sanitizer_common/TestCases/Linux/prctl.cpp:45:9 in main
Shadow bytes around the buggy address:
  0x7f28a4cffd80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x7f28a4cffe00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x7f28a4cffe80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x7f28a4cfff00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x7f28a4cfff80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x7f28a4d00000: f1 f1 f1 f1 00 f2 f2 f2[04]f2 f8 f8 f8 f8 f8 f8
  0x7f28a4d00080: f8 f8 f8 f8 f8 f2 f2 f2 f2 f2 f8 f2 f2 f2 f8 f8
  0x7f28a4d00100: f3 f3 f3 f3 00 00 00 00 00 00 00 00 00 00 00 00
  0x7f28a4d00180: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x7f28a4d00200: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x7f28a4d00280: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07 
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
...

```

https://github.com/llvm/llvm-project/pull/101749