stdlib: Tests for mbrtoc8, c8rtomb, and the char8_t typedef.

This change adds tests for the mbrtoc8 and c8rtomb functions adopted for
C++20 via WG21 P0482R6 and for C2X via WG14 N2653, and for the char8_t
typedef adopted for C2X from WG14 N2653.

The tests for mbrtoc8 and c8rtomb specifically exercise conversion to
and from Big5-HKSCS because of special cases that arise with that encoding.
Big5-HKSCS defines some double byte sequences that convert to more than
one Unicode code point.  In order to test this, the locale dependencies
for running tests under wcsmbs is expanded to include zh_HK.BIG5-HKSCS.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>