cmd/internal/obj/arm: fix large stack offsets on nacl/arm

The code already fixed large non-stack offsets
but explicitly excluded stack references.
Perhaps you could get away with that before,
but current versions of nacl reject such stack
references. Rewrite them the same as the others.

For #11956 but probably not the last problem.

Change-Id: I0db4e3a1ed4f88ccddf0d30228982960091d9fb7
Reviewed-on: https://go-review.googlesource.com/13010Reviewed-by: Dave Cheney <dave@cheney.net>

src/cmd/internal/obj/arm/asm5.go

@@ -472,8 +472,8 @@ func asmoutnacl(ctxt *obj.Link, origPC int32, p *obj.Prog, o *Optab, out []uint3
 			break
 		}
 
-		if (p.To.Type == obj.TYPE_MEM && p.To.Reg != REG_R13 && p.To.Reg != REG_R9) || // MOVW Rx, X(Ry), y != 13 && y != 9
-			(p.From.Type == obj.TYPE_MEM && p.From.Reg != REG_R13 && p.From.Reg != REG_R9) { // MOVW X(Rx), Ry, x != 13 && x != 9
+		if (p.To.Type == obj.TYPE_MEM && p.To.Reg != REG_R9) || // MOVW Rx, X(Ry), y != 9
+			(p.From.Type == obj.TYPE_MEM && p.From.Reg != REG_R9) { // MOVW X(Rx), Ry, x != 9
 			var a *obj.Addr
 			if p.To.Type == obj.TYPE_MEM {
 				a = &p.To