XCOFF ld segfaults when running ld testsuite

XCOFF ld segfaults when running ld testsuite

The binutils XCOFF support doesn't handle random linker scripts very
well at all.  These tweaks to final_link fix segfaults when some
linker created sections are discarded due to "/DISCARD/ : { *(.*) }"
in scripts.  The xcoff_mark change is necessary to not segfault on
symbols defined in scripts, which may be bfd_link_hash_defined yet
have u.def.section set to bfd_und_section_ptr.  (Which might seem odd,
but occurs during early stages of linking before input sections are
mapped.)

	* xcofflink.c (xcoff_mark): Don't mark const sections.
	(bfd_xcoff_record_link_assignment): Add FIXME.
	(_bfd_xcoff_bfd_final_link): Don't segfault on assorted magic
	sections being discarded by linker script.