runtime: avoid inconsistent goroutine state in profiler

runtime: avoid inconsistent goroutine state in profiler

Because profiling signals can arrive at any time, we must
handle the case where a profiling signal arrives halfway
through a goroutine switch. Luckily, although there is much
to think through, very little needs to change.

Fixes #6000.
Fixes #6015.

R=golang-dev, dvyukov
CC=golang-dev
https://golang.org/cl/13421048