runtime: fix data race in stackalloc

Stack shrinking happens during mark phase,
and it assumes that it owns stackcache in mcache.
Stack cache flushing also happens during mark phase,
and it accesses stackcache's w/o any synchronization.
This leads to stackcache corruption:
http://goperfd.appspot.com/log/309af5571dfd7e1817259b9c9cf9bcf9b2c27610

LGTM=khr
R=khr
CC=golang-codereviews, rsc
https://golang.org/cl/126870043

src/pkg/runtime/stack.c

@@ -223,9 +223,11 @@ runtime·stackalloc(G *gp, uint32 n)
 			n2 >>= 1;
 		}
 		c = g->m->mcache;
-		if(c == nil) {
-			// This can happen in the guts of exitsyscall or
+		if(c == nil || g->m->gcing || g->m->helpgc) {
+			// c == nil can happen in the guts of exitsyscall or
 			// procresize. Just get a stack from the global pool.
+			// Also don't touch stackcache during gc
+			// as it's flushed concurrently.
 			runtime·lock(&stackpoolmu);
 			x = poolalloc(order);
 			runtime·unlock(&stackpoolmu);
@@ -285,7 +287,7 @@ runtime·stackfree(G *gp, void *v, Stktop *top)
 		}
 		x = (MLink*)v;
 		c = g->m->mcache;
-		if(c == nil) {
+		if(c == nil || g->m->gcing || g->m->helpgc) {
 			runtime·lock(&stackpoolmu);
 			poolfree(x, order);
 			runtime·unlock(&stackpoolmu);