cmd/gc: disallow pointer constants

Fixes issue 7760.

LGTM=iant
R=iant, remyoudompheng
CC=golang-codereviews
https://codereview.appspot.com/130720043

Committer: Ian Lance Taylor <iant@golang.org>
runtime: fix getgcmask
bv.data is an array of uint32s but the code was using
offsets computed for an array of bytes.
Add a test for stack GC info.
Fixes issue 8531.

LGTM=rsc
R=golang-codereviews
CC=golang-codereviews, khr, rsc
https://codereview.appspot.com/124450043
fmt: fix size returned on fast path of ReadRune
Fixes issue 8512.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://codereview.appspot.com/130090043