This patch implements the core language and compiler dependent library 
changes proposed in WG14 N2653 [1] for C2x. The changes include:
- Change of type for UTF-8 string literals from array of char to array
   of char8_t (unsigned char) when targeting C2x.
- A new atomic_char8_t typedef.
- A new ATOMIC_CHAR8_T_LOCK_FREE macro defined in terms of the existing
   __GCC_ATOMIC_CHAR8_T_LOCK_FREE predefined macro.

Tested on Linux x86_64.

gcc/ChangeLog:

2022-01-07  Tom Honermann  <tom@honermann.net>

	* ginclude/stdatomic.h (atomic_char8_t,
	ATOMIC_CHAR8_T_LOCK_FREE): New typedef and macro.

gcc/c/ChangeLog:

2022-01-07  Tom Honermann  <tom@honermann.net>

	* c-parser.c (c_parser_string_literal): Use char8_t as the type
	of CPP_UTF8STRING when char8_t support is enabled.
	* c-typeck.c (digest_init): Allow initialization of an array
	of character type by a string literal with type array of
	char8_t.

gcc/c-family/ChangeLog:

2022-01-07  Tom Honermann  <tom@honermann.net>

	* c-lex.c (lex_string, lex_charconst): Use char8_t as the type
	of CPP_UTF8CHAR and CPP_UTF8STRING when char8_t support is
	enabled.
	* c-opts.c (c_common_post_options): Set flag_char8_t if
	targeting C2x.

Tom.

[1]: 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