runtime: skip TestReturnAfterStackGrowInCallback if gcc is not found

Fixes #11754

Change-Id: Ifa423ca6eea46d1500278db290498724a9559d14
Reviewed-on: https://go-review.googlesource.com/12347
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/syscall_windows_test.go

@@ -578,6 +578,9 @@ func forceStackCopy() (r int) {
 }
 
 func TestReturnAfterStackGrowInCallback(t *testing.T) {
+	if _, err := exec.LookPath("gcc"); err != nil {
+		t.Skip("skipping test: gcc is missing")
+	}
 
 	const src = `
 #include <stdint.h>