runtime: fix goroutine stack accounting

Fixes #6166.
Fixes #6168.

R=golang-dev, bradfitz, remyoudompheng
CC=golang-dev
https://golang.org/cl/12927045

src/pkg/runtime/proc.c

@@ -1676,7 +1676,7 @@ runtime·malg(int32 stacksize)
 			stk = g->param;
 			g->param = nil;
 		}
-		g->stacksize = StackSystem + stacksize;
+		newg->stacksize = StackSystem + stacksize;
 		newg->stack0 = (uintptr)stk;
 		newg->stackguard = (uintptr)stk + StackGuard;
 		newg->stackguard0 = newg->stackguard;