[compiler-rt] [compiler-rt] Stop using x86 builtin on AArch64 with GCC (PR #93890)

[Simon Tatham via llvm-commits]

statham-arm wrote:

Ah, someone offline has pointed out to me that gcc only gained `__has_builtin` in gcc 10, so before that, you have to check based on the gcc version.

Elsewhere in the file there's a very similar check for `__builtin_fabs128` and `__builtin_fabsq`. Do those have the same availability in older gcc versions? Should that check be fixed in the same way as this one?

https://github.com/llvm/llvm-project/pull/93890