runtime: rename m.gcing to m.preemptoff and make it a string

m.gcing has become overloaded to mean "don't preempt this g" in
general.  Once the garbage collector is preemptible, the one thing it
*won't* mean is that we're in the garbage collector.

So, rename gcing to "preemptoff" and make it a string giving a reason
that preemption is disabled.  gcing was never set to anything but 0 or
1, so we don't have to worry about there being a stack of reasons.

Change-Id: I4337c29e8e942e7aa4f106fc29597e1b5de4ef46
Reviewed-on: https://go-review.googlesource.com/3660
Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/debug.go

@@ -24,13 +24,13 @@ func GOMAXPROCS(n int) int {
 
 	semacquire(&worldsema, false)
 	gp := getg()
-	gp.m.gcing = 1
+	gp.m.preemptoff = "GOMAXPROCS"
 	systemstack(stoptheworld)
 
 	// newprocs will be processed by starttheworld
 	newprocs = int32(n)
 
-	gp.m.gcing = 0
+	gp.m.preemptoff = ""
 	semrelease(&worldsema)
 	systemstack(starttheworld)
 	return ret