runtime: fix stacks_sys accounting

subtract freed count before span gets merged during free.

LGTM=dvyukov
R=dmitry, dvyukov
CC=golang-codereviews
https://codereview.appspot.com/112450045
build: be verbose when running tests on Plan 9

This is a temporary change to see how far the
builder gets when it times out.

LGTM=aram, 0intro
R=0intro, aram
CC=golang-codereviews, mischief
https://codereview.appspot.com/111400043

Committer: Aram Hăvărneanu <aram@mgk.ro>
sync/atomic: fix data races in tests
Fixes issue 8389.

LGTM=r
R=golang-codereviews, r
CC=golang-codereviews
https://codereview.appspot.com/111320044
cmd/gc: allocate select descriptor on stack

benchmark                      old ns/op     new ns/op     delta
BenchmarkSelectUncontended     220           165           -25.00%
BenchmarkSelectContended       209           161           -22.97%
BenchmarkSelectProdCons        1042          904           -13.24%

But more importantly this change will allow
to get rid of free function in runtime.

Fixes issue 6494.

LGTM=rsc, khr
R=golang-codereviews, rsc, dominik.honnef, khr
CC=golang-codereviews, remyoudompheng
https://codereview.appspot.com/107670043
cmd/gc: do not mark PC as pointer on stack
GC thinks that it's an invalid pointer on plan9:
http://build.golang.org/log/f7da638a20544ef7f45e81f23fdbc4875044c296

LGTM=0intro
R=golang-codereviews, 0intro
CC=aram, golang-codereviews
https://codereview.appspot.com/113350043
test: add test for confusion with dot imports

The gccgo compiler would fail this test.  The fix for gccgo is
http://codereview.appspot.com/116960043 .

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/118000043
test: avoid "declared but not used" errors in shift1.go

I'm improving gccgo's detection of variables that are only set
but not used, and it triggers additional errors on this code.
The new gccgo errors are correct; gc seems to suppress them
due to the other, expected, errors.  This change uses the
variables so that no compiler will complain.

gccgo change is http://codereview.appspot.com/119920043 .

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/116050043
make.bat: do not exit builder on successful completion of make.bat (fixes build)

LGTM=dvyukov
R=golang-codereviews, dvyukov
CC=golang-codereviews
https://codereview.appspot.com/118020043