gdb/ser-unix: add POSIX cfsetispeed/cfsetospeed support for custom baud rates

glibc 2.42 and later, and GNU Hurd, accept arbitrary baud rates
directly via cfsetispeed and cfsetospeed.  This behaviour is expected
to be standardized in a future POSIX revision.

Introduce a configure-time test (HAVE_CFSETSPEED_ARBITRARY) that
detects this capability and add a new platform-agnostic helper
set_custom_baudrate_posix.  The main dispatcher now prefers the POSIX
path when it is available, falling back to the existing Linux (BOTHER)
or Darwin (IOSSIOSPEED) implementations otherwise.

The HAVE_CUSTOM_BAUDRATE_SUPPORT guard is extended to also cover the
new POSIX case.

Suggested-by: Kevin Buettner <kevinb@redhat.com>
Suggested-by: Maciej W. Rozycki <macro@orcam.me.uk>
Signed-off-by: Sunil Dora <sunilkumar.dora@windriver.com>
Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Approved-by: Kevin Buettner <kevinb@redhat.com>