[clang] [llvm] [BPF] Add load-acquire and store-release instructions under -mcpu=v4 (PR #108636)

[Peilin Ye via cfe-commits]

peilin-ye wrote:

Sure Yonghong, I can include that in this PR.  Similarly, since we cannot have both `relaxed_load<atomic_load_zext_{8,16}>` and `relaxed_load<atomic_load_sext_{8,16}>`, I'll keep it `zext` (`BPF_MEM` | `BPF_LDX`) for now.  For example:
```c
int foo(char *ptr) {
    return __atomic_load_n(ptr, __ATOMIC_RELAXED);
}
```
This'll be compiled into:
```
0000000000000000 <foo>:
       0:	71 11 00 00 00 00 00 00	w1 = *(u8 *)(r1 + 0x0)
       1:	bc 10 08 00 00 00 00 00	w0 = (s8)w1
       2:	95 00 00 00 00 00 00 00	exit
```


https://github.com/llvm/llvm-project/pull/108636