diagnostics: special-case -fdiagnostics-text-art-charset=ascii for LANG=C

diagnostics: special-case -fdiagnostics-text-art-charset=ascii for LANG=C

In the LWN discussion of the "ASCII" art in GCC 14
  https://lwn.net/Articles/946733/#Comments
there was some concern about the use of non-ASCII characters in the
output.

Currently -fdiagnostics-text-art-charset defaults to "emoji".
To better handle older terminals by default, this patch special-cases
LANG=C to use -fdiagnostics-text-art-charset=ascii.

gcc/ChangeLog:
	* diagnostic.cc (diagnostic_initialize): When LANG=C, update
	default for -fdiagnostics-text-art-charset from emoji to ascii.
	* doc/invoke.texi (fdiagnostics-text-art-charset): Document the above.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>