runtime: do not shrink stacks GOCOPYSTACK=0

LGTM=rsc
R=golang-codereviews
CC=golang-codereviews, khr, rsc
https://golang.org/cl/76070043

src/pkg/runtime/stack.c

@@ -776,6 +776,8 @@ runtime·shrinkstack(G *gp)
 	uintptr used, oldsize, newsize;
 	MSpan *span;
 
+	if(!runtime·copystack)
+		return;
 	oldstk = (byte*)gp->stackguard - StackGuard;
 	oldbase = (byte*)gp->stackbase + sizeof(Stktop);
 	oldsize = oldbase - oldstk;