[native_toolchain_c] Add a `Language.objectiveC`

[dcharkes]

We have some custom logic for compiling Objective-C in

native/pkgs/ffigen/test/native_objc_test/setup.dart

Lines 20 to 27 in f5d27c5

	Future<String> _buildObject(String input) async {
	final output = '$input.o';
	await _runClang(['-x', 'objective-c', '-c', input, '-fpic'], output);
	return output;
	}
	Future<void> _linkLib(List<String> inputs, String output) =>
	_runClang(['-shared', '-framework', 'Foundation', ...inputs], output);

We could make it reusable by adding it to package:native_toolchain_c's CBuilder.