They correspond to x86's lock bt{s,r,c}, and the implementation calls them on x86/x86_64.

LLVM 14 and older don't support generating lock bt{s,r,c}.
https://godbolt.org/z/G1TMKza97
LLVM 15 only supports generating lock bt{s,r,c} for immediate bit offsets.
https://godbolt.org/z/dzzhr81z6
LLVM 16 can generate lock bt{s,r,c} for both immediate and register bit offsets.
#48 (comment)
So, use fetch_* based implementations on LLVM 16+, otherwise use asm based implementations.

Closes #48