Allow easier opt-out for `static_libgcc` feature

[cerisier]

cc @trybka @armandomontanez

When a rules-based toolchain enables static_link_cpp_runtimes (required to use static_runtime_lib / dynamic_runtime_lib), rules_cc currently also enables static_libgcc such that -static-libgcc is added to all link actions.

This is problematic for toolchains that:

• use clang with compiler-rt + libunwind (i.e., not GCC’s libgcc for runtime/unwinding), or
• expect a single, process-wide unwinder when DSOs throw/catch across boundaries.

Current escape hatch

--features=-static_libgcc works, but is not ideal for a public/toolchain-default experience.

The alternative option is to redefine/duplicate the entire experimental_replace_legacy_action_config_features feature set just to drop //cc/toolchains/args/static_libgcc:feature.

That duplication is especially painful because some targets aren’t publicly visible, e.g.:

• :backfill_legacy_args
• //cc/toolchains/args/soname_flags:feature

Mitigation

I see a few ways to fix this:

1. Gate static_libgcc behind an explicit use_libgcc flag.
2. Make all targets under experimental_replace_legacy_action_config_features public so that opting out for a given toolchain is feasible.
3. Make static_libgcc opt-in for rules-based toolchains.

[armandomontanez]

We should definitely do (2).

If we ignore tradition (whether Google-specific or not) -static-libgcc seems like it should be a toolchain-specific implementation detail like -pthread. For that reason, I'd venture to say we should also do (3).

[cerisier]

Cheers for taking a look.

Do you want me to make a PR for 2 or 3 or both ? Or are there pending items before we can do these ?

[cerisier]

Also, -static-libgcc is not recognized by the clang driver for darwin targets.

$ echo 'int main() {}' | clang -x c - -static-libgcc -target aarch64-apple-macosx
clang: error: unsupported option '-static-libgcc'

So we should also either add it conditionally (unless the feature becomes opt-in)

Note that @platforms//os:macos is not enough since the option is not recognized for the whole darwin family (ios etc.)
I guess this is another argument for making it opt-in.

[cerisier]

@trybka any thoughts on @armandomontanez thoughts ? I'm willing to provide the PRs for that