cmd/6g: relax constraint on variables that need zeroing.

On amd64p32 pointers are 32-bit-aligned and cannot be assumed to
have an offset multiple of widthreg. Instead check that they are
withptr-aligned.

Also change the threshold for region merging to 2*widthreg
instead of 2*widthptr because performance on amd64 and amd64p32
is expected to be the same.

Fixes #7712.

LGTM=khr
R=rsc, dave, khr, brad, bradfitz
CC=golang-codereviews
https://golang.org/cl/84690044

src/cmd/6g/ggen.c

@@ -42,12 +42,12 @@ defframe(Prog *ptxt)
 			continue;
 		if(n->class != PAUTO)
 			fatal("needzero class %d", n->class);
-		if(n->type->width % widthreg != 0 || n->xoffset % widthreg != 0 || n->type->width == 0)
+		if(n->type->width % widthptr != 0 || n->xoffset % widthptr != 0 || n->type->width == 0)
 			fatal("var %lN has size %d offset %d", n, (int)n->type->width, (int)n->xoffset);
 
-		if(lo != hi && n->xoffset + n->type->width >= lo - 2*widthptr) {
+		if(lo != hi && n->xoffset + n->type->width >= lo - 2*widthreg) {
 			// merge with range we already have
-			lo = n->xoffset;
+			lo = rnd(n->xoffset, widthreg);
 			continue;
 		}
 		// zero old range