[bzl/build] Specify clang-common config

[nezdolik]

When building locally cilium-envoy, cilium-envoy-starter via make install for linux amd64/arm64, compilation failed with:

clang-18: error: unsupported option '-pass-exit-codes'

this is because overriden clang version is not being propagated to the build:

ARGO_BAZEL_REPIN=true  bazel  build --platforms=//bazel:linux_aarch64 --config=release //:cilium-envoy

[jrajahalme]

IMO this would be better added to .bazelrc, right after importing envoy.bazelrc. Should also figure out if we need anything else from the generated clang.bazelrc file where this was already.

[nezdolik]

not sure i fully understand/agree. We still need to tell bazel to use clang config when building cilium-envoy-proxy (by default it will use gcc). bazelrc file just contains different build configs. Or did you mean to include clang config into release config? E.g.

build:release --config=clang

[nezdolik]

but also may need some of those:

build:clang --action_env='PATH=${PATH}' --host_action_env='PATH=${PATH}'
build:clang --action_env='LLVM_CONFIG=${LLVM_PREFIX}/bin/llvm-config' --host_action_env='LLVM_CONFIG=${LLVM_PREFIX}/bin/llvm-config'
build:clang --repo_env='LLVM_CONFIG=${LLVM_PREFIX}/bin/llvm-config'
build:clang --linkopt='-L$(llvm-config --libdir)'
build:clang --linkopt='-Wl,-rpath,$(llvm-config --libdir)'

[jrajahalme]

It is OK to make all builds use clang, IMO. We no longer test any builds with GCC.

Either keep the current clang setup script and amend the Makefiles to use it on all targets, or remove it and figure out what of the generated clang.bazelrc has to be moved to .bazelrc.

[nezdolik]

I will proceed with option 1 now (less effort). Envoy has switched to hermetic toolchain in https://github.com/envoyproxy/envoy/pull/39679/changes, this is smth we would benefit from as well.