[clang] [compiler-rt] [AArch64] Implement __builtin_cpu_supports, compiler-rt tests. (PR #82378)

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eaeltsin wrote:

Thanks @DanielKristofKiss - considering compilations with older toolchains is a good reason to choose the particular behavior. It would be very valuable if all these reasons and the selected behavior for unsupported builtins/parameters are documented, i.e. here - https://clang.llvm.org/docs/LanguageExtensions.html#builtin-functions


https://github.com/llvm/llvm-project/pull/82378