alpha: make XTABS equivalent to TAB3

XTABS is an old name for "expand tabs to spaces" flag, which was a
separate flag on some BSD implementations. POSIX, however, specifies
that this effect is enabled with TAB3 output mode.

Currently, alpha is the only architecture that has the value of
the XTABS flag not equivalent to TAB3.

Signed-off-by: Eugene Syromiatnikov <esyr@redhat.com>
Signed-off-by: Matt Turner <mattst88@gmail.com>