libatomic: perform write before context switch

libatomic: perform write before context switch

Atomic actions block a thread at the "switch_to_master" function call, so under
the current structure (which isn't quite fit for relaxed modeling yet...) we
should perform the memory write before calling "switch_to_master".

If not, we can observe sequences like the following, where x is an atomic
variable. All actions are seq_cst:

 Initially, x = 0

 Thread    Action
 ------    ------
 1         r1 = x;       // r1 = 0
 1          x = r1 + 1;  //  x = 1, not stored yet?
 2         r2 = x;       // r2 = 0
 2          x = r2       //  x = 1, not stored yet?

Then, depending on scheduling, Thread 1 or Thread 2 might complete first, with
its write being performed *after* it receives control again.