[libcxx][CI] Work around Arm buildkite failures

[DavidSpickett]

Summary

For reasons unknown, the build is now using compilers
from /usr/bin instead of /usr/local/bin which is where
we have our clang-12 aliases placed.

[DavidSpickett]

Working build:

[437/523] /usr/local/bin/c++  -DNDEBUG -D_FILE_OFFSET_BITS=64 -D_GNU_SOURCE...

Bad build:

[412/521] /usr/bin/c++  -DNDEBUG -D_FILE_OFFSET_BITS=64 -D_GNU_SOURCE ...
c++: error: unrecognized command line option '--target=armv7-linux-gnueabihf'

Our /usr/bin/ is a gcc which is A: not what we want and B: doesn't need a target option so it throws it out.

The recent libcxx changes seem fine but we did have a container restart about 12hr ago so this issue could have been lingering and brought up by using a fresh cmake config. I will find the actual reason.

[DavidSpickett]

@ldionne This fixes the Arm bots so if you want them green today you can merge this.

I'll find out what's really going on on Monday.

[ldionne]

Thanks for the workaround. Let's try to find out what's going on and fix it properly once we've put the fire out.

[DavidSpickett]

I found the issue and the next release of the buildkite agent will have a fix https://github.com/buildkite/agent/issues/1464.

I'll revert this once it's available. (your other uses of /usr/bin/cc for the other bots should continue to work it's just /usr/local that was effected)

[ldionne]

In D105704#2873389, @DavidSpickett wrote:

I found the issue and the next release of the buildkite agent will have a fix https://github.com/buildkite/agent/issues/1464.

I'll revert this once it's available. (your other uses of /usr/bin/cc for the other bots should continue to work it's just /usr/local that was effected)

Thanks a lot for following up on this and fixing the Buildkite issue!

libcxx/utils/ci/run-buildbot

legacy-standalone)
 
     echo "+++ Running the libc++ tests"
     ${NINJA} -vC "${BUILD_DIR}/libcxx" check-cxx
 
     echo "+++ Running the libc++abi tests"
     ${NINJA} -vC "${BUILD_DIR}/libcxxabi" check-cxxabi
 ;;
 aarch64)
+    export CC=/usr/local/bin/cc
+    export CXX=/usr/local/bin/c++
     clean
     generate-cmake -C "${MONOREPO_ROOT}/libcxx/cmake/caches/AArch64.cmake"
     check-cxx-cxxabi
 ;;
 aarch64-noexceptions)
+    export CC=/usr/local/bin/cc
+    export CXX=/usr/local/bin/c++
     clean
     generate-cmake -C "${MONOREPO_ROOT}/libcxx/cmake/caches/AArch64.cmake" \
     -DLIBCXX_ENABLE_EXCEPTIONS=OFF \
     -DLIBCXXABI_ENABLE_EXCEPTIONS=OFF
     check-cxx-cxxabi
 ;;
 # Aka Armv8 32 bit
 armv8)
+    export CC=/usr/local/bin/cc
+    export CXX=/usr/local/bin/c++
     clean
     generate-cmake -C "${MONOREPO_ROOT}/libcxx/cmake/caches/Armv8Arm.cmake"
     check-cxx-cxxabi
 ;;
 armv8-noexceptions)
+    export CC=/usr/local/bin/cc
+    export CXX=/usr/local/bin/c++
     clean
     generate-cmake -C "${MONOREPO_ROOT}/libcxx/cmake/caches/Armv8Thumb-noexceptions.cmake"
     check-cxx-cxxabi
 ;;
 # Armv7 32 bit. One building Arm only one Thumb only code.
 armv7)
+    export CC=/usr/local/bin/cc
+    export CXX=/usr/local/bin/c++
     clean
     generate-cmake -C "${MONOREPO_ROOT}/libcxx/cmake/caches/Armv7Arm.cmake"
     check-cxx-cxxabi
 ;;
 armv7-noexceptions)
+    export CC=/usr/local/bin/cc
+    export CXX=/usr/local/bin/c++
     clean
     generate-cmake -C "${MONOREPO_ROOT}/libcxx/cmake/caches/Armv7Thumb-noexceptions.cmake"
     check-cxx-cxxabi
 ;;
 windows-dll)
     clean
     # TODO: Currently, building with the experimental library breaks running
     # tests (the test linking look for the c++experimental library with the