XML feature description support.

XML feature description support.

	* NEWS: Mention target descriptions, "set tdesc filename",
	"unset tdesc filename", "show tdesc filename", and
	qXfer:features:read.
	* arch-utils.c (choose_architecture_for_target): New function.
	(gdbarch_info_fill): Call it.
	* target-descriptions.c (struct property): Make members non-const.
	(struct target_desc): Add arch member.
	(target_description_filename): New variable.
	(target_find_description): Try via XML first.
	(tdesc_architecture): New.
	(free_target_description, make_cleanup_free_target_description): New.
	(set_tdesc_property): Call xstrdup.
	(set_tdesc_architecture, tdesc_set_cmdlist, tdesc_show_cmdlist)
	(tdesc_unset_cmdlist, unset_tdesc_cmd, unset_tdesc_filename_cmd)
	(set_tdesc_cmd, show_tdesc_cmd, set_tdesc_filename_cmd)
	(show_tdesc_filename_cmd, _initialize_target_descriptions): New.
	* target-descriptions.h (tdesc_architecture)
	(make_cleanup_free_target_description, set_tdesc_architecture): New
	prototypes.
	* Makefile.in (SFILES): Add xml-tdesc.c.
	(COMMON_OBS): Add xml-tdesc.o.
	(target-descriptions.o): Update.
	(xml-tdesc.o): New rule.
	* xml-tdesc.c, xml-tdesc.h: New files.
	* remote.c (PACKET_qXfer_features): New enum.
	(remote_protocol_features): Add qXfer:features:read.
	(remote_xfer_partial): Handle TARGET_OBJECT_AVAILABLE_FEATURES.
	(_initialize_remote): Register qXfer:features:read.
	* target.h (enum target_object): Add TARGET_OBJECT_AVAILABLE_FEATURES.
	* features/gdb-target.dtd: New file.

	* linux-i386-low.c (the_low_target): Set arch_string.
	* linux-x86-64-low.c (the_low_target): Likewise.
	* linux-low.c (linux_arch_string): New.
	(linux_target_ops): Add it.
	* linux-low.h (struct linux_target_ops): Add arch_string.
	* server.c (write_qxfer_response): Use const void * for DATA.
	(get_features_xml): New.
	(handle_query): Handle qXfer:features:read.  Report it for qSupported.
	* target.h (struct target_ops): Add arch_string method.

	* gdb.texinfo (Target Descriptions): New section.
	(General Query Packets): Add QPassSignals anchor.  Mention
	qXfer:features:read under qSupported.  Expand mentions of
	qXfer:memory-map:read and QPassSignals.  Document
	qXfer:features:read.