compiler-rt build failure on 20.1.5 coming from 20.1.3, on both x86_64 & armv7l (linux)

[satmandu]

A bunch of errors like this happen before the build fails.

[285*10/480] Building CXX object compiler-rt/lib/xray/CMakeFiles/RTXray.x86_64.dir/xray_buffer_queue.cpp.o
FAILED: compiler-rt/lib/xray/CMakeFiles/RTXray.x86_64.dir/xray_buffer_queue.cpp.o
/usr/local/tmp/crew/llvm20_build.20250524020233.dir/builddir/bin/clang++ --target=x86_64-cros-linux-gnu -DSANITIZER_COMMON_NO_REDEFINE_BUILTINS -DXRAY_HAS_EXCEPTIONS=1 -DXRAY_SUPPORTED=1 -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -D__func__=__FUNCTION__ -I/usr/local/tmp/crew/llvm20_build.20250524020233.dir/compiler-rt/lib/xray/.. -I/usr/local/tmp/crew/llvm20_build.20250524020233.dir/compiler-rt/lib/xray/../../include -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wno-comment -Wstring-conversion -fdiagnostics-color -ffunction-sections -fdata-sections -O3 -DNDEBUG -std=c++17 -O3 -MD -MT compiler-rt/lib/xray/CMakeFiles/RTXray.x86_64.dir/xray_buffer_queue.cpp.o -MF compiler-rt/lib/xray/CMakeFiles/RTXray.x86_64.dir/xray_buffer_queue.cpp.o.d -o compiler-rt/lib/xray/CMakeFiles/RTXray.x86_64.dir/xray_buffer_queue.cpp.o -c /usr/local/tmp/crew/llvm20_build.20250524020233.dir/compiler-rt/lib/xray/xray_buffer_queue.cpp
In file included from /usr/local/tmp/crew/llvm20_build.20250524020233.dir/compiler-rt/lib/xray/xray_buffer_queue.cpp:14:
/usr/local/tmp/crew/llvm20_build.20250524020233.dir/compiler-rt/lib/xray/xray_buffer_queue.h:21:10: fatal error: 'cstddef' file not found
   21 | #include <cstddef>
      |          ^~~~~~~~~
1 error generated.

The build is using these options:

            -DCOMPILER_RT_BUILD_BUILTINS=ON \
            -DCOMPILER_RT_BUILD_LIBFUZZER=OFF \
            -DCOMPILER_RT_BUILD_SANITIZERS=OFF \
            -DCOMPILER_RT_DEFAULT_TARGET_ONLY=ON \
            -DLLVM_ENABLE_RUNTIMES=compiler-rt

This ends up succeeding on i686 though.