compare_and_exchange

Compares atomic to oldval and, if equal, sets it to newval.

If atomic was not equal to oldval then no change occurs.

This compare and exchange is done atomically.

Think of this operation as an atomic version of `{ if (*atomic == oldval) { *atomic = newval; return TRUE; } else return FALSE; }`.

This call acts as a full compiler and hardware memory barrier.

While atomic has a `volatile` qualifier, this is a historical artifact and the pointer passed to it should not be `volatile`.

Parameters:

atomic	a pointer to a pointer-sized value
oldval	the value to compare with
newval	the value to conditionally replace with

Returns:

true if the exchange took place