cmd/gc: make bitmaps shorter

cmd/gc: make bitmaps shorter

Sort non-pointer-containing data to the low end of the
stack frame, and make the bitmaps only cover the
pointer-containing top end.

Generates significantly less garbage collection bitmap
for programs with large byte buffers on the stack.

Only 2% shorter for godoc, but 99.99998% shorter
in some test cases.

Fixes arm build.

TBR=golang-dev
CC=cshapiro, golang-dev, iant
https://golang.org/cl/12541047