Add Hermetic C++ Toolchains for Linux x86_64 builds.

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Add Hermetic C++ Toolchains for Linux x86_64 builds.

Hermetic toolchains provide builds that are isolated from the host system, cutting down on unexpected dependencies and side effects.

By default, JAX now builds for Linux x86_64 architectures (both CPU and CUDA-enabled GPU) using hermetic C++ toolchains. For non-hermetic builds, add the flag --config=clang_local. For remote builds with a non-hermetic toolchain, simply append _clang_local to the existing RBE flag. For example, if hermetic RBE build runs with --config=rbe_linux_cpu, the non-hermetic version would be --config=rbe_linux_cpu_clang_local.

Example 1: Run CPU tests for Linux x86_64

For hermetic tests run the command for Linux x86_64 build without env variables CC, CXX, etc.:

bazel test //tests:cpu_tests \
	--config=avx_posix \
	--config=clang \
	--linkopt=-lrt \
	--host_linkopt=-lrt \
	--repo_env=HERMETIC_PYTHON_VERSION="3.13" \
	--test_tag_filters=-multiaccelerator

For non-hermetic tests use commands with the flag "--config=clang_local" and env variables CC, CXX, etc.:

bazel test //tests:cpu_tests \
	--config=clang_local \
	--config=avx_posix \
	--config=clang \
	--linkopt=-lrt \
	--host_linkopt=-lrt \
	--repo_env=HERMETIC_PYTHON_VERSION="3.13" \
	--test_tag_filters=-multiaccelerator \
	--action_env=CLANG_COMPILER_PATH=/usr/lib/llvm-18/bin/clang \
	--repo_env=CC=/usr/lib/llvm-18/bin/clang \
	--repo_env=CXX=/usr/lib/llvm-18/bin/clang++ \
	--repo_env=BAZEL_COMPILER=/usr/lib/llvm-18/bin/clang \
	--action_env=CCC_OVERRIDE_OPTIONS="^--gcc-install-dir=/usr/lib/gcc/x86_64-linux-gnu/13"

Example 2: Run CUDA GPU tests for Linux x86_64

For hermetic tests, run the command without env variables CC, CXX, etc.:
bazel test //tests:gpu_tests \
	--config=cuda \
	--config=clang \
	--config=build_cuda_with_nvcc \
	--config=avx_posix \
	--linkopt=-lrt \
	--host_linkopt=-lrt \
	--test_tag_filters=-multiaccelerator \
	--repo_env=HERMETIC_PYTHON_VERSION=3.13

For non-hermetic tests use commands with the flag "--config=clang_local" and env variables CC, CXX, etc.:

bazel test //tests:gpu_tests \
	--config=clang_local \
	--config=avx_posix \
	--config=build_cuda_with_nvcc \
	--config=clang \
	--config=cuda \
	--linkopt=-lrt \
	--host_linkopt=-lrt \
	--action_env=CCC_OVERRIDE_OPTIONS="^--gcc-install-dir=/usr/lib/gcc/x86_64-linux-gnu/9" \
	--test_tag_filters=-multiaccelerator \
	--action_env=CLANG_COMPILER_PATH=/usr/lib/llvm-18/bin/clang \
	--repo_env=CC=/usr/lib/llvm-18/bin/clang \
	--repo_env=CXX=/usr/lib/llvm-18/bin/clang++ \
	--repo_env=BAZEL_COMPILER=/usr/lib/llvm-18/bin/clang \
	--repo_env=HERMETIC_PYTHON_VERSION=3.13

Example 3: Run RBE build for Linux x86_64

For hermetic build, run the same command as before modification:
bazel build \
    --config=ci_linux_x86_64 \
    --repo_env=HERMETIC_PYTHON_VERSION=3.13 \
    //jaxlib/tools:jaxlib_wheel

For non-hermetic builds, add "_clang_local" suffix to RBE configuration:
bazel build \
    --config=ci_linux_x86_64_clang_local \
    --repo_env=HERMETIC_PYTHON_VERSION=3.13 \
    //jaxlib/tools:jaxlib_wheel

We don't support other "rbe_*_clang_local" configuration flags, and you could construct your own combinations.