testsuite: skip objc-hi/objcxx-hi when cross compiling

objc-hi/objcxx-hi should be skipped when cross compiling. The existing
opsys('darwin') predicate only asserts the host system is darwin but
tells us nothing about the target, hence the oversight.

testsuite/tests/driver/objc/all.T

 test('objc-hi',
      [ unless(opsys('darwin'), skip),
+       when(config.target_wrapper is not None, skip),
        objc_src,
        expect_fail_for(['ghci']) ],
      compile_and_run, ['-framework Foundation'])
 
 test('objcxx-hi',
      [ unless(opsys('darwin'), skip),
+       when(config.target_wrapper is not None, skip),
        objcxx_src,
        expect_fail_for(['ghci']) ],
      compile_and_run, ['-framework Foundation -lc++'])