PR libstdc++/81266 fix std::thread::native_handle_type test

PR libstdc++/81266 fix std::thread::native_handle_type test

The test uses remove_pointer because in most cases native_handle_type is
a pointer to the actual type that the C++ class contains. However, for
std::thread, native_handle_type is the same type as the type contained
in std::thread, and so remove_pointer is not needed. On targets where
pthread_t is a pointer type remove_pointer<native_handle_type> is not a
no-op, instead it transforms pthread_t and causes the test to fail.

The fix is to not apply remove_pointer when testing std::thread.

Backport from mainline
2019-05-10  Jonathan Wakely  <jwakely@redhat.com>

	PR libstdc++/81266
	* testsuite/util/thread/all.h: Do not use remove_pointer for
	std::thread::native_handle_type.

From-SVN: r271094