misc/cgo/test: add test for cgo callbacks with different amount of stack space available

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/11677043

misc/cgo/test/callback_c.c

@@ -57,3 +57,10 @@ twoSleep(int n)
 	BackgroundSleep(n);
 	return mysleep(n);
 }
+
+void
+callGoStackCheck(void)
+{
+	extern void goStackCheck(void);
+	goStackCheck();
+}

misc/cgo/test/cgo_test.go

@@ -43,5 +43,6 @@ func Test5337(t *testing.T)                { test5337(t) }
 func Test5548(t *testing.T)                { test5548(t) }
 func Test5603(t *testing.T)                { test5603(t) }
 func Test3250(t *testing.T)                { test3250(t) }
+func TestCallbackStack(t *testing.T)       { testCallbackStack(t) }
 
 func BenchmarkCgoCall(b *testing.B) { benchCgoCall(b) }