Add basic char*_t support for libc (partial WG14 N2653) (#90360)

This PR implements a part of WG14 N2653:
 - Define C23 char8_t
 - Define C11 char16_t
 - Define C11 char32_t
 
 Missing goals are:
- The type of UTF-8 character literals is changed from unsigned char to
char8_t. (Since UTF-8 character literals already have type unsigned
char, this is not a semantic change).
- New mbrtoc8() and c8rtomb() functions declared in <uchar.h> enable
conversions between multibyte characters and UTF-8.
    - A new ATOMIC_CHAR8_T_LOCK_FREE macro.
    - A new atomic_char8_t typedef name.