runtime: fix gctrace=1

updatememstats is called on both the m and g stacks.
Call into flushallmcaches correctly.  flushallmcaches
can only run on the M stack.

This is somewhat temporary.  once ReadMemStats is in
Go we can have all of this code M-only.

LGTM=dvyukov
R=golang-codereviews, dvyukov
CC=golang-codereviews
https://golang.org/cl/116880043

src/pkg/runtime/mgc0.c

@@ -2172,7 +2172,10 @@ runtime·updatememstats(GCStats *stats)
 	}
 
 	// Flush MCache's to MCentral.
-	runtime·mcall(flushallmcaches_m);
+	if(g == g->m->g0)
+		flushallmcaches();
+	else
+		runtime·mcall(flushallmcaches_m);
 
 	// Aggregate local stats.
 	cachestats();