bfd: ensure that symbols targeted by DWARF relocations are kept in XCOFF

This patch improves XCOFF garbage collector pass, in order to keep
symbols being referenced only by special sections like DWARF sections.

bfd/
	* xcofflink.c (xcoff_mark): Replace SEC_MARK by gc_mark.
	Look through relocations even if xcoff_section_data is NULL.
	(xcoff_sweep): Check if any sections of a file is kept before
	adding its special sections.
	Call xcoff_mark for special sessions being kept instead of just
	marking them.
	(SEC_MARK): Remove
	(xcoff_mark_symbol): Replace SEC_MARK by gc_mark.
	(xcoff_keep_symbol_p): Likewise.
	(bfd_xcoff_size_dynamic_sections): Likewise.
	(xcoff_find_tc0): Likewise.