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.
