m68k: Revive _TIF_* masks

While the core m68k code does not use the _TIF_* masks anymore, there
exists generic code that relies on their presence.  Fortunately none of
that code is used on m68k, currently.

Re-add the various _TIF_* masks, which were removed in commit
cddafa35 ("m68k/m68knommu: merge MMU and non-MMU
thread_info.h"), to avoid future nasty surprises.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Greg Ungerer <gerg@linux-m68k.org>
Link: https://lore.kernel.org/r/20200826122923.22821-1-geert@linux-m68k.org