Delete IRIX support

This does most of the mechanical removal.  IOW, the easy part.

This doesn't touch procfs.c as that'd be a harder excision,
potentially affecting Solaris.

mips-tdep.c is left alone.  E.g., I didn't delete the GDB_OSABI_IRIX
enum value, nor references to it in mips-tdep.c.  Some comments
mentioning IRIX ABIs may still be relevant and I wouldn't know what to
do with them. in That can always be done on a separate pass,
preferably by someone who can test on MIPS.

I didn't remove a reference to IRIX in testsuite/lib/future.exp, as I
believe that code is imported from DejaGNU.

Built and tested on x86_64 Fedora 20, with --enable-targets=all.

Tested that building for --target=mips-sgi-irix6 on x86_64 Fedora 20
fails with:

 checking for default auto-load directory... $debugdir:$datadir/auto-load
 checking for default auto-load safe-path... $debugdir:$datadir/auto-load
 *** Configuration mips-sgi-irix6 is obsolete.
 *** Support has been REMOVED.
 make[1]: *** [configure-gdb] Error 1
 make[1]: Leaving directory `/home/pedro/gdb/mygit/build-irix'
 make: *** [all] Error 2

gdb/
2014-10-10  Pedro Alves  <palves@redhat.com>

	* Makefile.in (ALL_TARGET_OBS): Remove mips-irix-tdep.o and solib-irix.o.
	(ALLDEPFILES): Remove mips-irix-tdep.c and solib-irix.c.
	(HFILES_NO_SRCDIR): Remove solib-irix.h.
	* NEWS: Mention that support for mips-sgi-irix5* mips-sgi-irix6*
	and been removed.
	* config/mips/irix5.mh, config/mips/irix6.mh: Delete files.
	* configure.ac: Remove references to IRIX.
	* configure.host: Add *-*-irix* to the obsolete hosts section.
	Remove all other references to irix.
	* irix5-nat.c, mips-irix-tdep.c, solib-irix.c, solib-irix.h:
	Delete files.

gdb/testsuite/
2014-10-10  Pedro Alves  <palves@redhat.com>

	* gdb.base/bigcore.exp: Remove references to IRIX.
	* gdb.base/funcargs.exp: Likewise.
	* gdb.base/interrupt.exp: Likewise.
	* gdb.base/mips_pro.exp: Likewise.
	* gdb.base/nodebug.exp: Likewise.
	* gdb.base/setvar.exp: Likewise.
	* lib/gdb.exp (gdb_compile_shlib): Remove mips-sgi-irix* case.