runtime: speed up cgo calls

runtime: speed up cgo calls

Allocate Defer on stack during cgo calls, as suggested
by dvyukov. Also includes some comment corrections.

benchmark                   old,ns/op   new,ns/op
BenchmarkCgoCall                  669         330
(Intel Xeon CPU 1.80GHz * 4, Linux 386)

R=dvyukov, rsc
CC=golang-dev
https://golang.org/cl/4910041