[llvm] Use 32 bit atomic on freebsd mips

[adrian]

Summary

gcc for mips32 doesn't include libatomic and doesn't generate
atomic sequences; so this just fails to link.

Instead, just use 32 bit for mips32/mips64.

(I haven't found a nice way to detect 32 or 64 bit mips here
without other header pollution, but it should be fine for freebsd/mips.)

[dim]

I think this is fine for now. Upstream will want to centralize the define somewhere in __config, but we can take that discussion there.