runtime: pass setmg function to cgo_init

This keeps the logic about how to set the thread-local variables
m and g in code compiled and linked by the gc toolchain,
an important property for upcoming cgo changes.

It's also just a nice cleanup: one less place to update when
these details change.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/7560048
doc: add summary of net/http additions to Go 1.1

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/7812050