MN10300: Deprecate gdbstub

Deprecate the MN10300 arch's gdbstub in favour of KGDB, which is more capable
in some areas (such as SMP) and almost as capable in others (it's I/O is not as
decoupled and it can't start as early).  gdbstub will be removed in a later
version when we're satisfied with KGDB's working.

Signed-off-by: David Howells <dhowells@redhat.com>