runtime: fix deadlock when gctrace

Calling ReadMemStats which does stoptheworld on m0 holding locks
was not a good idea.
Stoptheworld holding locks is a recipe for deadlocks (added check for this).
Stoptheworld on g0 may or may not work (added check for this as well).
As far as I understand scavenger will print incorrect numbers now,
as stack usage is not subtracted from heap. But it's better than deadlocking.

LGTM=khr
R=golang-codereviews, rsc, khr
CC=golang-codereviews, rlh
https://golang.org/cl/124670043

src/pkg/runtime/malloc.go

@@ -413,6 +413,7 @@ func gogc(force int32) {
 		return
 	}
 	releasem(mp)
+	mp = nil
 
 	if panicking != 0 {
 		return
@@ -441,7 +442,11 @@ func gogc(force int32) {
 	startTime := gonanotime()
 	mp = acquirem()
 	mp.gcing = 1
+	releasem(mp)
 	stoptheworld()
+	if mp != acquirem() {
+		gothrow("gogc: rescheduled")
+	}
 
 	clearpools()
 
@@ -474,6 +479,7 @@ func gogc(force int32) {
 	semrelease(&worldsema)
 	starttheworld()
 	releasem(mp)
+	mp = nil
 
 	// now that gc is done, kick off finalizer thread if needed
 	if !concurrentSweep {