tweak pcs in traceback so they point to calling line instead of line after call.

R=rsc
DELTA=2  (0 added, 0 deleted, 2 changed)
OCL=19745
CL=19745

src/runtime/print.c

@@ -32,7 +32,7 @@ void
 sys·printpc(void *p)
 {
 	prints("PC=0x");
-	sys·printpointer(sys·getcallerpc(p));
+	sys·printpointer((byte*)sys·getcallerpc(p) - 1);	// -1 to get to CALL instr.
 }
 
 void

src/runtime/rt2_amd64.c

@@ -70,7 +70,7 @@ traceback(uint8 *pc, uint8 *sp, void* r15)
 
 		/* print this frame */
 		prints("0x");
-		sys·printpointer(callpc);
+		sys·printpointer(callpc  - 1);	// -1 to get to CALL instr.
 		prints("?zi\n");
 		prints("\t");
 		prints(name);