Implement and document --gc-keep-exported

Implement and document --gc-keep-exported

include/
	* bfdlink.h (struct bfd_link_info): Add gc_keep_exported.
bfd/
	* elflink.c (bfd_elf_gc_mark_dynamic_ref_symbol): Add handling
	for info->gc_keep_exported.
	(bfd_elf_gc_sections): Likewise.
ld/
	* ld.texinfo: Document --gc-keep-exported.
	* ldlex.h (enum option_values): Add OPTION_GC_KEEP_EXPORTED.
	* lexsup.c (parse_args): Add handling for --gc-keep-exported.