sim: watchpoints: use common sim_pc_get

Few arches implement STATE_WATCHPOINTS()->pc while all of them implement
sim_pc_get.  Lets switch the sim-watch core for monitoring pc events to
the sim_pc_get API so this module works for all ports, and then we can
delete this old back channel of snooping in the port's cpu state -- the
code needs the pointer to the pc storage so that it can read out bytes
and compare them to the watchrange.

This also fixes the logic on multi-cpu sims by removing the limitation
of only being able to watch CPU0's state.

sim/mips/interp.c

@@ -355,7 +355,6 @@ sim_open (SIM_OPEN_KIND kind, host_callback *cb,
   cpu = STATE_CPU (sd, 0); /* FIXME */
 
   /* FIXME: watchpoints code shouldn't need this */
-  STATE_WATCHPOINTS (sd)->pc = &(PC);
   STATE_WATCHPOINTS (sd)->interrupt_handler = interrupt_event;
 
   /* Initialize the mechanism for doing insn profiling.  */