runtime: skip TestStdcallAndCDeclCallbacks when gcc is missing

Fixes #10167.

Change-Id: Ib6c6b2b5dde47744b69f65482a21964fa3c12090
Reviewed-on: https://go-review.googlesource.com/7600Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

src/runtime/syscall_windows_test.go

@@ -436,6 +436,9 @@ var cbTests = []cbTest{
 }
 
 func TestStdcallAndCDeclCallbacks(t *testing.T) {
+	if _, err := exec.LookPath("gcc"); err != nil {
+		t.Skip("skipping test: gcc is missing")
+	}
 	tmp, err := ioutil.TempDir("", "TestCDeclCallback")
 	if err != nil {
 		t.Fatal("TempDir failed: ", err)