vt: make use of ucs_get_fallback() when glyph is unavailable

vt: make use of ucs_get_fallback() when glyph is unavailable

Attempt to display a fallback character when given character doesn't
have an available glyph. The substitution may not be as good as the
original character but still way more helpful than a squared question
mark.

Example substitutions: À -> A, ç -> c, ø -> o, ─ -> -, © -> C, etc.

See gen_ucs_fallback_table.py for a comprehensive list.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>
Link: https://lore.kernel.org/r/20250507141535.40655-8-nico@fluxnic.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>