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.

(cherry picked from commit 595c0894)

testsuite/tests/driver/objc/all.T

@@ -8,12 +8,14 @@ skip_if_not_osx = if_not_platform(['i386-apple-darwin','x86_64-apple-darwin'], s
 
 test('objc-hi',
      [ skip_if_not_osx,
+       when(config.target_wrapper is not None, skip),
        objc_src,
        expect_fail_for(['ghci']) ],
      compile_and_run, ['-framework Foundation'])
 
 test('objcxx-hi',
      [ skip_if_not_osx,
+       when(config.target_wrapper is not None, skip),
        objcxx_src,
        expect_fail_for(['ghci']) ],
      compile_and_run, ['-framework Foundation -lc++'])