go.tools/go/loader: fix Solaris build

Also fix one test that failed on Plan 9.

LGTM=0intro, dave
R=golang-codereviews, 0intro, dave
CC=golang-codereviews
https://golang.org/cl/101370053

go/loader/stdlib_test.go

@@ -138,8 +138,9 @@ func TestStdlib(t *testing.T) {
 }
 
 func TestCgoOption(t *testing.T) {
-	if runtime.GOOS == "windows" {
-		// on windows, the net and os/user packages don't use cgo.
+	switch runtime.GOOS {
+	// on these systems, the net and os/user packages don't use cgo.
+	case "plan9", "solaris", "windows":
 		return
 	}
 	// Test that we can load cgo-using packages with