Use relaxed atomics to access the monitor line.

We must ensure that accesses occur, they do not have any other
compiler-visible effects. Bruce found some situations where
optimization could remove an access, and provided a patch to use
volatile qualifier for the state variables. Since volatile behaviour
there is the compiler-specific interpretation of the keyword, use
relaxed atomics instead, which gives exactly the desired semantic.

Noted by and discussed with: bde
Sponsored by: The FreeBSD Foundation
MFC after: 1 week