stdlib: Implement mbrtoc8, c8rtomb, and the char8_t typedef.

This change provides implementations for the mbrtoc8 and c8rtomb
functions adopted for C++20 via WG21 P0482R6 and for C2X via WG14
N2653.  It also provides the char8_t typedef from WG14 N2653.

The mbrtoc8 and c8rtomb functions are declared in uchar.h in C2X
mode or when the _GNU_SOURCE macro or C++20 __cpp_char8_t feature
test macro is defined.

The char8_t typedef is declared in uchar.h in C2X mode or when the
_GNU_SOURCE macro is defined and the C++20 __cpp_char8_t feature
test macro is not defined (if __cpp_char8_t is defined, then char8_t
is a builtin type).

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

1 files changed, 3 insertions, 0 deletions

diff --git a/wcsmbs/Versions b/wcsmbs/Versions
index 0b31c1b..ec28acf 100644
--- a/wcsmbs/Versions
+++ b/wcsmbs/Versions
@@ -49,4 +49,7 @@ libc {
     wcstof32; wcstof64; wcstof32x;
     wcstof32_l; wcstof64_l; wcstof32x_l;
   }
+  GLIBC_2.36 {
+    c8rtomb; mbrtoc8;
+  }
 }