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

[Peilin Ye via cfe-commits]

peilin-ye wrote:

Pushed v8 to make it generate plain (`BPF_MEM`) loads and stores if user requested `__ATOMIC_RELAXED`, as suggested by Yonghong.  Updated commit message accordingly.

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