Handle surrogate pairs in c16rtomb (bug 23794, DR#488, C2X).

The c16rtomb implementation has:

  // XXX The ISO C 11 spec I have does not say anything about handling
  // XXX surrogates in this interface.

The DR#488 resolution, as applied to C2X, requires surrogate pairs to
be handled here (so the first call returns 0 and stores the high
surrogate in the mbstate_t, while the second call combines the
surrogates, produces a multibyte character and returns the number of
bytes written).  This patch implements that.  (mbrtoc16 already
handled producing surrogates as output.)

Tested for x86_64.

	[BZ #23794]
	* wcsmbs/c16rtomb.c (c16rtomb): Save first character of surrogate
	pair and return 0 in that case, and use saved character to
	interpret following character.
	* wcsmbs/tst-c16-surrogate.c: New file.
	* wcsmbs/Makefile (tests): Add tst-c16-surrogate.c.
	[$(run-built-tests) = yes] ($(objpfx)tst-c16-surrogate.out):
	Depend on $(gen-locales)