runtime: Fix build on older ARM

        The ARM external linking CL used BLX instructions in gcc assembler. Replace with BL to retain support on older ARM processors.

R=rsc
CC=golang-dev
https://codereview.appspot.com/12938043

Committer: Russ Cox <rsc@golang.org>
net: make TCPStress test shorter
It timeouts on freebsd builders:
http://build.golang.org/log/3d8169e13bff912bebf6fd3c54b34ad2d29a7221
but there are always runnable goroutines,
which suggests that it's slowly progressing.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/12939043
runtime: implement SysUnused on windows
Fixes issue 5584.

R=golang-dev, chaishushan, alex.brainman
CC=golang-dev
https://codereview.appspot.com/12720043