gdb: update "info threads" output when no threads match the arguments

If "info threads" is provided with the thread ID argument but no such
threads matching the thread ID(s) are found, GDB prints

  No threads match '<ID...>'.

Update this output to the more generalized

  No threads matched.

The intention is that the next patch, and potentially future ones,
will extend the command with more filter/match arguments.  We cannot
customize the output to each such argument.  Hence, be more generic.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Approved-by: Pedro Alves <pedro@palves.net