cmd/gc: fix stkptrsize calculation

I moved the pointer block from one end of the frame
to the other toward the end of working on the last CL,
and of course that made the optimization no longer work.

Now it works again:

0030 (bug361.go:12) DATA    gclocals·0+0(SB)/4,$4
0030 (bug361.go:12) DATA    gclocals·0+4(SB)/4,$3
0030 (bug361.go:12) GLOBL   gclocals·0+0(SB),8,$8

Fixes arm build (this time for sure!).

TBR=golang-dev
CC=cshapiro, golang-dev, iant
https://golang.org/cl/12627044