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

[Peilin Ye via cfe-commits]

peilin-ye wrote:

Pushed v9:
 - Rewrote `acquire-release.ll` to make it also test `__ATOMIC_RELAXED` (renamed to `atomic-load-store.ll`, just like e.g. `llvm/test/CodeGen/X86/atomic-load-store.ll`)
 - Style change as suggested by Matt

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