Support atomic CAS on BPF targets

[taiki-e]

Another target that doesn't support atomic CAS in the standard library.

portable-atomic/tools/build.sh

Line 199 in d547b60

bpf*) ;; # TODO

This LLVM test indicates that many of the 64-bit and 32-bit atomic operations are already supported.

Refs:

• https://github.com/taiki-e/atomic-maybe-uninit/blob/HEAD/src/arch/README.md#bpf
• https://www.kernel.org/doc/html/latest/bpf/instruction-set.html#atomic-operations

[taiki-e]

This will be resolved by rust-lang/rust#105708.

[taiki-e]

rust-lang/rust#105708 merged.

[taiki-e]

rust-lang/rust#105708 is not actually working properly.