PDP11: Added support for PC values to be displayed in debug output.

This required a simulator specific implementation since the PDP11 PC register isn't stored in a normal memory location.  It is loaded from a temporary location upon simulator instruction execution startup (and saved to that location when instuction execution stops).  In order to reference the PC value while executing instructions (for debug output), this extended access model is required.

PDP11/pdp11_cpu.c

@@ -630,12 +630,19 @@ DEVICE cpu_dev = {
     NULL, &cpu_set_size, NULL
     };
 
+t_value pdp11_pc_value (void)
+{
+return (t_value)PC;
+}
+
 t_stat sim_instr (void)
 {
 int abortval, i;
 volatile int32 trapea;                                  /* used by setjmp */
 t_stat reason;
 
+sim_vm_pc_value = &pdp11_pc_value;
+
 /* Restore register state
 
         1. PSW components