[llvm] [Support] Prefer AUX vector for page size (PR #126863)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-solaris11-sparcv9` running on `solaris11-sparcv9` while building `llvm` at step 4 "build stage 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/13/builds/5292

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

```
Step 4 (build stage 1) failure: 'ninja -j8' (failure)
...
[148/5583] Building CXX object lib/Support/CMakeFiles/LLVMSupport.dir/raw_ostream.cpp.o
[149/5583] Building CXX object lib/Support/CMakeFiles/LLVMSupport.dir/Errno.cpp.o
[150/5583] Building CXX object lib/Support/CMakeFiles/LLVMSupport.dir/DynamicLibrary.cpp.o
[151/5583] Building CXX object lib/Support/CMakeFiles/LLVMSupport.dir/YAMLParser.cpp.o
[152/5583] Building CXX object lib/Support/CMakeFiles/LLVMSupport.dir/RWMutex.cpp.o
[153/5583] Building CXX object lib/Support/CMakeFiles/LLVMSupport.dir/Valgrind.cpp.o
[154/5583] Building CXX object lib/Support/CMakeFiles/LLVMSupport.dir/Watchdog.cpp.o
[155/5583] Building CXX object lib/Support/CMakeFiles/LLVMSupport.dir/Memory.cpp.o
[156/5583] Building C object lib/Support/BLAKE3/CMakeFiles/LLVMSupportBlake3.dir/blake3_dispatch.c.o
[157/5583] Building CXX object lib/Support/CMakeFiles/LLVMSupport.dir/Process.cpp.o
FAILED: lib/Support/CMakeFiles/LLVMSupport.dir/Process.cpp.o 
/opt/llvm/19/bin/clang++ -DGTEST_HAS_RTTI=0 -D_DEBUG -D_FILE_OFFSET_BITS=64 -D_GLIBCXX_ASSERTIONS -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/opt/llvm-buildbot/home/solaris11-sparcv9/clang-solaris11-sparcv9/stage1/lib/Support -I/opt/llvm-buildbot/home/solaris11-sparcv9/clang-solaris11-sparcv9/llvm/llvm/lib/Support -I/opt/llvm-buildbot/home/solaris11-sparcv9/clang-solaris11-sparcv9/stage1/include -I/opt/llvm-buildbot/home/solaris11-sparcv9/clang-solaris11-sparcv9/llvm/llvm/include -I/opt/llvm-buildbot/home/solaris11-sparcv9/clang-solaris11-sparcv9/llvm/llvm/include/llvm/Support/Solaris -fPIC -fno-semantic-interposition -fvisibility-inlines-hidden -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -pedantic -Wno-long-long -Wc++98-compat-extra-semi -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wsuggest-override -Wstring-conversion -Wmisleading-indentation -Wctad-maybe-unsupported -fdiagnostics-color -ffunction-sections -fdata-sections -Werror=global-constructors  -O -DNDEBUG -UNDEBUG -std=c++17  -fno-exceptions -funwind-tables -fno-rtti -MD -MT lib/Support/CMakeFiles/LLVMSupport.dir/Process.cpp.o -MF lib/Support/CMakeFiles/LLVMSupport.dir/Process.cpp.o.d -o lib/Support/CMakeFiles/LLVMSupport.dir/Process.cpp.o -c /opt/llvm-buildbot/home/solaris11-sparcv9/clang-solaris11-sparcv9/llvm/llvm/lib/Support/Process.cpp
In file included from /opt/llvm-buildbot/home/solaris11-sparcv9/clang-solaris11-sparcv9/llvm/llvm/lib/Support/Process.cpp:122:
/opt/llvm-buildbot/home/solaris11-sparcv9/clang-solaris11-sparcv9/llvm/llvm/lib/Support/Unix/Process.inc:70:34: error: no member named 'getauxval' in the global namespace
   70 |   static const int page_size = ::getauxval(AT_PAGESZ);
      |                                ~~^
1 error generated.
[158/5583] Building C object lib/Support/BLAKE3/CMakeFiles/LLVMSupportBlake3.dir/blake3.c.o
[159/5583] Building C object lib/Support/BLAKE3/CMakeFiles/LLVMSupportBlake3.dir/blake3_portable.c.o
[160/5583] Building CXX object lib/Support/CMakeFiles/LLVMSupport.dir/Threading.cpp.o
[161/5583] Building CXX object lib/Support/CMakeFiles/LLVMSupport.dir/Program.cpp.o
[162/5583] Building CXX object lib/Support/CMakeFiles/LLVMSupport.dir/Signals.cpp.o
[163/5583] Building CXX object lib/Support/CMakeFiles/LLVMSupport.dir/Path.cpp.o
[164/5583] Building CXX object lib/Support/CMakeFiles/LLVMSupport.dir/VirtualFileSystem.cpp.o
ninja: build stopped: subcommand failed.

```

</details>

https://github.com/llvm/llvm-project/pull/126863