[llvm] BPF: Ensure __sync_fetch_and_add() always generate atomic_fetch_add insn (PR #101428)

[Bryce Kahle via llvm-commits]

brycekahle wrote:

@yonghong-song this seems to be a breaking change for folks still targeting `v1` or `v2` instruction sets. LLVM not generates an instruction that isn't available.

https://github.com/llvm/llvm-project/pull/101428