cmd/6g: fix stack zeroing preamble on amd64p32.

It was using a REP STOSQ but putting in CX the number of 32-bit
words to clear.

LGTM=dave
R=rsc, dave
CC=golang-codereviews
https://golang.org/cl/75240043

src/cmd/6g/ggen.c

@@ -30,11 +30,11 @@ defframe(Prog *ptxt)
 	// when it looks for pointers.
 	p = ptxt;
 	if(stkzerosize > 0) {
-		p = appendpp(p, AMOVQ, D_CONST, 0, D_AX, 0);	
-		p = appendpp(p, AMOVQ, D_CONST, stkzerosize/widthptr, D_CX, 0);	
-		p = appendpp(p, ALEAQ, D_SP+D_INDIR, frame-stkzerosize, D_DI, 0);	
+		p = appendpp(p, movptr, D_CONST, 0, D_AX, 0);	
+		p = appendpp(p, movptr, D_CONST, stkzerosize/widthptr, D_CX, 0);	
+		p = appendpp(p, leaptr, D_SP+D_INDIR, frame-stkzerosize, D_DI, 0);	
 		p = appendpp(p, AREP, D_NONE, 0, D_NONE, 0);	
-		appendpp(p, ASTOSQ, D_NONE, 0, D_NONE, 0);	
+		appendpp(p, stosptr, D_NONE, 0, D_NONE, 0);	
 	}
 }