Do not hardcode default/experimental c{pp}std

Prevent bugs like b/232866078 by using the same values across Bazel and
Soong

Test: bp2build.sh
Change-Id: If257f9f5f8e8a70bbf3a8cf5479758c703c25c3f

cc/compiler.go

@@ -295,8 +295,12 @@ func addToModuleList(ctx ModuleContext, key android.OnceKey, module string) {
 	getNamedMapForConfig(ctx.Config(), key).Store(module, true)
 }
 
+func useGnuExtensions(gnuExtensions *bool) bool {
+	return proptools.BoolDefault(gnuExtensions, true)
+}
+
 func maybeReplaceGnuToC(gnuExtensions *bool, cStd string, cppStd string) (string, string) {
-	if gnuExtensions != nil && *gnuExtensions == false {
+	if !useGnuExtensions(gnuExtensions) {
 		cStd = gnuToCReplacer.Replace(cStd)
 		cppStd = gnuToCReplacer.Replace(cppStd)
 	}