Remove support for FreeBSD/alpha.

Remove support for FreeBSD/alpha.

FreeBSD last shipped a release for Alpha (6.3) in 2008.
This also removes support for GNU/kFreeBSD on Alpha.

gdb/ChangeLog:

	* Makefile.in (ALL_64_TARGET_OBS): Remove alpha-fbsd-tdep.o.
	(ALLDEPFILES): Remove alpha-fbsd-tdep.c
	* NEWS: Mention that support for FreeBSD/alpha was removed.
	* alpha-fbsd-tdep.c: Delete file.
	* config/alpha/fbsd.mh: Delete file.
	* configure.host: Delete alpha*-*-freebsd* and
	alpha*-*-kfreebsd*-gnu.
	* configure.tgt: Delete alpha*-*-freebsd* and
	alpha*-*-kfreebsd*-gnu.