dtrace: Make pid provider work on trivial tests

'newpc' needs set in the "common" case.  With this, the trivial test

 $ dtrace -n 'pid$target:libc:strlen:entry { trace(timestamp); }' -p
 <pid>

now works.

MFC after:	3 weeks

sys/cddl/contrib/opensolaris/uts/powerpc/dtrace/fasttrap_isa.c

@@ -486,6 +486,10 @@ fasttrap_pid_probe(struct trapframe *frame)
 			rp->lr = rp->pc + 4;
 		break;
 	case FASTTRAP_T_COMMON:
+		curthread->t_dtrace_pc = pc;
+		curthread->t_dtrace_npc = pc + 4;
+		curthread->t_dtrace_on = 1;
+		new_pc = pc;
 		break;
 	};
 done: