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

[Peilin Ye via cfe-commits]

peilin-ye wrote:

When landing, please simply land this PR as-is - considering that I've made several references to this PR on the mailing list, and the two other commits except the "main" one are pretty small, I no longer think it's worth it to restructure this into stacked PRs.  For future contributions, I'll make sure to use stacked PRs from the beginning and avoid squashing/force-pushing.  Thanks!
- - -
Pushed v11:
 - Rebased, `ninja check-all` passed
 - Changed encoding to `BPF_LOAD_ACQ=0x100`, `BPF_STORE_REL=0x110` per [mailing list discussion](https://lore.kernel.org/bpf/Z7AxcSwD-topj1bk@google.com/)


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