re PR libstdc++/66417 (std::codecvt_utf16<char32_t, 0x10FFFF, std::codecvt_mode::generate_header> generates incorrect output)

	PR libstdc++/66417
	* src/c++11/codecvt.cc (write_utf16_code_point): Use adjust_byte_order
	for single UTF-16 units.
	* testsuite/22_locale/codecvt/codecvt_utf16/66417.cc: New.

From-SVN: r224217