Implement mutexes in non-pthreads

[curiousdannii]

From rust-lang/rust#156350

Implement actual locking mutexes even in non-pthreads builds.

[sbc100]

Thanks @curiousdannii. I actually started looking into this already.

I'm trying to find a way to do this that doesn't regress codesize. I think we can figure it out.