runtime: increase stack system space on windows/amd64

gotest src/pkg/exp/template/html was crashing because the exception handler overflowed the goroutine stack.

R=alex.brainman, golang-dev
CC=golang-dev
https://golang.org/cl/5031049

src/pkg/runtime/stack.h

@@ -58,7 +58,7 @@ enum {
 	// purposes like signal handling. Used on Windows because
 	// it does not use a separate stack.
 #ifdef __WINDOWS__
-	StackSystem = 2048,
+	StackSystem = 512 * sizeof(uintptr),
 #else
 	StackSystem = 0,
 #endif