Drop support for Mercury on IRIX.

IRIX itself is more or less dead and the Mercury support for it has not been
tested for many years.

configure.ac:
compiler/notes/overall_design.m:
doc/user_guide.texi:
scripts/mgnuc.in:
	As above.

NEWS:
	Announce the above.
Delete files resurrected by the conversion to git.

scripts/ml.sh:
scripts/mmake.sh:
	As above.
Minor rewording of README.{AIX,HPUX}.

README.AIX:
	As above.

README.HPUX:
	As above, plus fix a cut-and-paste error.
Drop support for OSF/1.

configure.ac:
	Drop support for OSF/1.

	Delete more IRIX stuff.

configure.ac:
runtime/mercury_conf.h.in:
runtime/mercury_thread.h:
	Delete workarounds required when using the Digital Unix pthreads
	library.

Mmake.common.in:
runtime/mercury_wrapper.c:
	Delete stuff for IRIX.

README.OSF1:
	Delete this file.

.README.in:
	Delete the pointers to the readme files for IRIX and OSF/1.

NEWS:
	Announce the dropping of support for OSF/1.