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

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eddyz87 wrote:

> I think it should be okay. Based on practice of selftest, feature testing sounds a good way to guard specific features. And in the long run, e.g., after llvm24 released, we do not even care to use feature guard as users can upgrade to llvm22/23/24.

Yes, but if someone upgrades to new llvm he would have to either adjust build scripts to disable `gotox`, or upgrade the kernel version. Just want to clarify if that's what we want.

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