cmd/gc: enforce register-width alignment for stack sizes.

This fixes the following amd64p32 issue:
    pkg/time/format.go:724: internal compiler error: twobitwalktype1: invalid initial alignment, Time

caused by the pointer zone ending on a 32-bit-aligned boundary.

LGTM=rsc
R=rsc, dave
CC=golang-codereviews
https://golang.org/cl/72270046

src/cmd/gc/pgen.c

@@ -429,8 +429,8 @@ allocauto(Prog* ptxt)
 		n->stkdelta = -stksize - n->xoffset;
 	}
 	stksize = rnd(stksize, widthreg);
-	stkptrsize = rnd(stkptrsize, widthptr);
-	stkzerosize = rnd(stkzerosize, widthptr);
+	stkptrsize = rnd(stkptrsize, widthreg);
+	stkzerosize = rnd(stkzerosize, widthreg);
 
 	fixautoused(ptxt);