[compiler-rt] [cmake][compiler-rt] Define _DEFAULT_SOURCE instead of enabling extensions (PR #163377)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-ppc64le-linux-multistage` running on `ppc64le-clang-multistage-test` while building `compiler-rt` at step 5 "ninja check 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/76/builds/13263

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

```
Step 5 (ninja check 1) failure: 1200 seconds without output running [b'ninja', b'check-all'], attempting to kill
...
PASS: SanitizerCommon-ubsan-powerpc64le-Linux :: Linux/signal_segv_handler.cpp (103111 of 103127)
PASS: SanitizerCommon-msan-powerpc64le-Linux :: Linux/signal_segv_handler.cpp (103112 of 103127)
PASS: SanitizerCommon-lsan-powerpc64le-Linux :: Linux/signal_segv_handler.cpp (103113 of 103127)
PASS: mlgo-utils :: corpus/extract_ir_script.test (103114 of 103127)
PASS: SanitizerCommon-tsan-powerpc64le-Linux :: Linux/signal_segv_handler.cpp (103115 of 103127)
PASS: SanitizerCommon-asan-powerpc64le-Linux :: Linux/signal_segv_handler.cpp (103116 of 103127)
PASS: LLVM :: tools/llvm-readobj/ELF/AMDGPU/elf-headers.test (103117 of 103127)
PASS: LLVM :: tools/llvm-objdump/ELF/AMDGPU/subtarget.ll (103118 of 103127)
PASS: lit :: shtest-define.py (103119 of 103127)
PASS: LLVM :: tools/llvm-readobj/ELF/note-core.test (103120 of 103127)
command timed out: 1200 seconds without output running [b'ninja', b'check-all'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=7606.132008
Step 11 (ninja check 2) failure: 1200 seconds without output running [b'ninja', b'check-all'], attempting to kill
...
[1268/1277] Linking CXX executable tools/clang/tools/extra/clangd/unittests/ClangdTests
[1269/1277] Building CXX object tools/clang/unittests/Format/CMakeFiles/FormatTests.dir/FormatTest.cpp.o
[1270/1277] Building CXX object tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/ASTMatchers/ASTMatchersNarrowingTest.cpp.o
[1271/1277] Linking CXX executable tools/clang/unittests/Format/FormatTests
[1272/1277] Building CXX object tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/AST/ASTImporterTest.cpp.o
[1273/1277] Building CXX object unittests/ADT/CMakeFiles/ADTTests.dir/SmallVectorTest.cpp.o
[1274/1277] Linking CXX executable unittests/ADT/ADTTests
[1275/1277] Building CXX object tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/ASTMatchers/ASTMatchersTraversalTest.cpp.o
[1276/1277] Linking CXX executable tools/clang/unittests/AllClangUnitTests
[1276/1277] Running all regression tests
command timed out: 1200 seconds without output running [b'ninja', b'check-all'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=1933.301277

```

</details>

https://github.com/llvm/llvm-project/pull/163377