cmd/5g: fix bit mask for div/mod routines clobbering R12

This patch is based only on reading the code.  I have not
tried to construct a test case.

Fixes #9077.

LGTM=minux
R=minux
CC=golang-codereviews
https://golang.org/cl/172110043

src/cmd/5g/reg.c

@@ -230,7 +230,7 @@ regopt(Prog *firstp)
 
 		/* the mod/div runtime routines smash R12 */
 		if(p->as == ADIV || p->as == ADIVU || p->as == AMOD || p->as == AMODU)
-			r->set.b[z] |= RtoB(12);
+			r->set.b[0] |= RtoB(12);
 	}
 	if(firstr == R)
 		return;