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

[via llvm-commits]

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yonghong-song wrote:

Thanks. Indeed simplified CHECKs a little bit.
For the difference for
```
; CHECK: r2 = atomic_fetch_add((u32 *)(r1 + 0), r2)
; CHECK-V3: w2 = atomic_fetch_add((u32 *)(r1 + 0), w2)
```
I think it is okay, since this is asm output from llc.

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