Otherwise the next goroutine run on the m
can get inadvertently locked if it executes a cgo call
that turns on the internal lock.

While we're here, fix the cgo panic unwind to
decrement m->ncgo like the non-panic unwind does.

Fixes #4971.

R=golang-dev, iant, dvyukov
CC=golang-dev
https://golang.org/cl/7627043