Remove obsolete IRIX 6.5 support

[Richard Sandiford]

OK from my point of view, except...

Rainer Orth <ro@CeBiTec.Uni-Bielefeld.DE> writes:
> 	* config/mips/mips.h (TARGET_GPWORD): Remove IRIX 6 N64 handling.
> 	(TARGET_IRIX6): Remove.
> 	(TARGET_CPU_CPP_BUILTINS): Remove IRIX 6 handling.
> 	Don't define LANGUAGE_C, _LANGUAGE_C for Objective-C.

...looking at SVN history, defining _LANGUAGE_C* for Objective-C
predates the Irix comment, so I think we'd better keep it.  How about
changing the comment to:

     /* Bizarre, but retained for backwards compatibility.  */

Thanks for all your work on this port, and for putting up with those
long bootstrap times :-)

Richard