libstdc++: Add "ASCII" as an alias for std::text_encoding::id::ASCII

As noted in LWG 4043, "ASCII" is not an alias for any known registered
character encoding, so std::text_encoding("ASCII").mib() == id::other.
Add the alias "ASCII" to the implementation-defined superset of aliases
for that encoding.

libstdc++-v3/ChangeLog:

	* include/bits/text_encoding-data.h: Regenerate.
	* scripts/gen_text_encoding_data.py: Add extra_aliases dict
	containing "ASCII".
	* testsuite/std/text_encoding/cons.cc: Check "ascii" is known.

Co-authored-by: Ewan Higgs <ewan.higgs@gmail.com>
Signed-off-by: Ewan Higgs <ewan.higgs@gmail.com>