This macro was added as part of moving std::endian from <type_traits> to
<bit>.

	* include/std/bit (__cpp_lib_endian): Define.
	* include/std/version (__cpp_lib_endian): Define.
	* testsuite/26_numerics/endian/2.cc: New.
	* testsuite/26_numerics/endian/3.cc: New.
	* testsuite/26_numerics/endian/4.cc: New.

From-SVN: r273828