Support on-demand global register allocation by passing on

	Support on-demand global register allocation by passing on
	base-plus-offset relocs to the linker.
	* config/tc-mmix.c: Tweak and fix typos in comments.
 	(allocate_undefined_gregs_in_linker): New variable.
	(OPTION_LINKER_ALLOCATED_GREGS): New option macro.
	(md_longopts): Add --linker-allocated-gregs.
	(md_parse_option) <case 'x'>: Imply --linker-allocated-gregs.
	<case OPTION_LINKER_ALLOCATED_GREGS>: New.
	(md_show_usage): Update text for -x.  Add text for
	--linker-allocated-gregs.
	(tc_gen_reloc): Derive default value for addend from val and
	baddsy.  Use addsec and bfd_is_abs_section in more places.  Don't
	emit error for BFD_RELOC_MMIX_BASE_PLUS_OFFSET without suitable
	GREG if allocate_undefined_gregs_in_linker.
	* doc/as.texinfo (Overview) <Target MMIX options>: Add
	--linker-allocated-gregs.
	* doc/c-mmix.texi (MMIX-Opts): Add blurb about
	--linker-allocated-gregs.  Mention that it's implied by -x.
	(MMIX-Pseudos) <GREG>: Mention when and how a GREG can be omitted.
	(MMIX-mmixal): Clarify dated comparison and location of MMIXware.

	* config/tc-mmix.h (md_parse_name): Use ISUPPER, not isupper.