BPF: Ensure __sync_fetch_and_add() always generate atomic_fetch_add insn (#101428)

Peilen Ye reported an issue ([1]) where for __sync_fetch_and_add(...)
without return value like
  __sync_fetch_and_add(&foo, 1);
llvm BPF backend generates locked insn e.g.
  lock *(u32 *)(r1 + 0) += r2

If __sync_fetch_and_add(...) returns a value like
  res = __sync_fetch_and_add(&foo, 1);
llvm BPF backend generates like
  r2 = atomic_fetch_add((u32 *)(r1 + 0), r2)

The above generation of 'lock *(u32 *)(r1 + 0) += r2' caused a problem
in jit since proper barrier is not inserted.

The above discrepancy is due to commit [2] where it tries to maintain
backward compatability since before commit [2],
__sync_fetch_and_add(...) generates lock insn in BPF backend.

Based on discussion in [1], now it is time to fix the above discrepancy
so we can have proper barrier support in jit. This patch made sure that
__sync_fetch_and_add(...) always generates atomic_fetch_add(...) insns.
Now 'lock *(u32 *)(r1 + 0) += r2' can only be generated by inline asm. I
also removed the whole BPFMIChecking.cpp file whose original purpose is
to detect and issue errors if XADD{W,D,W32} may return a value used
subsequently. Since insns XADD{W,D,W32} are all inline asm only now,
such error detection is not needed.

[1]
https://lore.kernel.org/bpf/ZqqiQQWRnz7H93Hc@google.com/T/#mb68d67bc8f39e35a0c3db52468b9de59b79f021f
[2]
https://github.com/llvm/llvm-project/commit/286daafd65129228e08a1d07aa4ca74488615744

Co-authored-by: Yonghong Song <yonghong.song@linux.dev>