cmd/gc: add way to specify 'noescape' for extern funcs

A new comment directive //go:noescape instructs the compiler
that the following external (no body) func declaration should be
treated as if none of its arguments escape to the heap.

Fixes issue 4099.

R=golang-dev, dave, minux.ma, daniel.morsing, remyoudompheng, adg, agl, iant
CC=golang-dev
https://codereview.appspot.com/7289048