[compiler-rt] compiler-rt: Exclude sync_fetch_and_* for any pre-ARMv6 targets (PR #139411)

[Ties Stuij via llvm-commits]

https://github.com/stuij commented:

Sorry for the late reply.

This patch certainly does feel a bit hackish but I do also think that since this patch is to the point, is harmless for non-v4/v5, and a more thorough rewrite feels like overkill, it LGTM, bar a bit of comment cleanup. See inline.

https://github.com/llvm/llvm-project/pull/139411