[clang] [llvm] [BPF] Add load-acquire and store-release instructions under -mcpu=v5 (PR #108636)

[Peilin Ye via cfe-commits]

peilin-ye wrote:

(pushed v2 to resolve the easier issues first :-)

- deleted redundant `CPU.empty()` check
- for now, fall back to `ACQUIRE` and `RELEASE` if user requested weaker memory orders (`RELAXED` or `CONSUME`), until we actually support them
  - took Eduard's suggestion in #107343 to use `foreach`, thanks!

I couldn't find a way to generate a better error message for `__ATOMIC_SEQ_CST`, however; it seems that CodeGen simply calls `CannotYetSelect()` if nothing in `MatcherTable` matches.  Any suggestions?

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