[clang] [Clang][BPF] Add __BPF_FEATURE_GOTOX (PR #165456)

[via cfe-commits]

4ast wrote:

> I think we come to a point where we need to decide if `gotox` is a part of `cpuv4`. If it is, we should keep it disabled by default, to avoid backwards compatibility issues. @4ast , @yonghong-song

it's fine for gotox to be in cpuv4. It will be generated only for large-ish switches where backward compat is unlikely to be an issue, and for explicit computed goto which would have failed to compile.

https://github.com/llvm/llvm-project/pull/165456