[PATCH 0/3]: C++20 P0482R6 and C2X N2653: support for char8_t, mbrtoc8(), and c8rtomb().

[Tom Honermann]

This series of patches provides the following:
- A fix for bug 25744 [1].
- Implementations of the mbrtoc8 and c8rtomb functions adopted for
   C++20 via WG21 P0482R6 [2] and for C2X via WG14 N2653 [3].
- A char8_t typedef as adopted for C2X via WG14 N2653 [3].

These patches addresses feedback provided in response to a previous 
submission [4].

Patch 1: A fix and test for bug 25744 [1].
Patch 2: Definitions of the mbrtoc8 and c8rtomb functions and the
          char8_t typedef.
Patch 3: Tests for the mbrtoc8 and c8rtomb functions and the char8_t
          typedef.

The fix for bug 25744 [1] is included in this patch series because the 
tests for mbrtoc8 and c8rtomb depend on it for exercising the special 
case where a pair of Unicode code points is converted to/from a single 
double byte sequence. Such conversion cases exist for Big5-HKSCS.

N2653 was adopted by WG14 for C2X during their recent meeting. This 
patch series enables the new declarations in C2X mode and when 
_GNU_SOURCE is defined.

Thank you to Joseph Myers and Carlos O'Donell for their prior reviews of 
this patch series.

Tom.

[1]: Bug 25744
      "mbrtowc with Big5-HKSCS returns 2 instead of 1 when consuming the
      second byte of certain double byte characters"
      https://sourceware.org/bugzilla/show_bug.cgi?id=25744

[2]: WG21 P0482R6
      "char8_t: A type for UTF-8 characters and strings (Revision 6)"
      https://wg21.link/p0482r6

[3]: WG14 N2653
      "char8_t: A type for UTF-8 characters and strings (Revision 1)"
      http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2653.htm

[4]: "[PATCH 2/3]: C++20 P0482R6 and C2X N2653: Implement mbrtoc8,
      c8rtomb, char8_t"
      https://sourceware.org/pipermail/libc-alpha/2022-February/136558.html