Fix alignment bugs in std::codecvt_utf16

	* src/c++11/codecvt.cc (range): Add non-type template parameter and
	define oerloaded operators for reading and writing code units.
	(range<Elem, false>): Define partial specialization for accessing
	wide characters in potentially unaligned byte ranges.
	(ucs2_span(const char16_t*, const char16_t*, ...))
	(ucs4_span(const char16_t*, const char16_t*, ...)): Change parameters
	to range<const char16_t, false> in order to avoid unaligned reads.
	(__codecvt_utf16_base<char16_t>::do_out)
	(__codecvt_utf16_base<char32_t>::do_out)
	(__codecvt_utf16_base<wchar_t>::do_out): Use range specialization for
	unaligned data to avoid unaligned writes.
	(__codecvt_utf16_base<char16_t>::do_in)
	(__codecvt_utf16_base<char32_t>::do_in)
	(__codecvt_utf16_base<wchar_t>::do_in): Likewise for writes. Return
	error if there are unprocessable trailing bytes.
	(__codecvt_utf16_base<char16_t>::do_length)
	(__codecvt_utf16_base<char32_t>::do_length)
	(__codecvt_utf16_base<wchar_t>::do_length): Pass arguments of type
	range<const char16_t, false> to span functions.
	* testsuite/22_locale/codecvt/codecvt_utf16/misaligned.cc: New test.

From-SVN: r246245