These functions won't ever unwind. This is useful for MemorySanitizer as it simplifies handling __atomic_load in particular.
I don't think we can necessarily guarantee argmemonly/readonly/writeonly, particularly since these library calls can take a lock somewhere inside. I'll definitely add nocapture and willreturn though.
Added willreturn. I think nocapture is probably best left to a future change since it's more invasive -- it would require keeping tabs of which arguments to each function is a pointer arg.