speakup: Add /dev/synthu device

speakup: Add /dev/synthu device

/dev/synth has always been 8bit, but applications nowadays mostly
expect to be using utf-8 encoding.  This adds /dev/synthu to be able
to synthesize non-latin1 characters.  This however remains limited
to 16bit unicode like the rest of speakup.  Any odd input or input
beyond 16bit is just discarded.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
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Link: https://lore.kernel.org/r/20240204155825.ditstifsbqndnce3@begin
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>