restore build with --enable-maintainer-mode

A build with --enable-maintainer-mode is currently failing with:

make[4]: *** No rule to make target '<SRC>/gas/config/te-ia64aix.h',
	 needed by '<SRC>/gas/po/gas.pot'.  Stop.
make[4]: Leaving directory '<$OBJ>/gas/po'
make[3]: *** [Makefile:1695: all-recursive] Error 1
...

As config/te-ia64aix.h is now removed, remove the corresponding fragment
from the makefile.

gas/
        * Makefile.am: Remove config/te-ia64aix.h.
        * Makefile.in: Regenerate.
        * po/POTFILES.in: Regenerate.
3 files changed