* target-descriptions.h (tdesc_compatible_p): New.
(tdesc_add_compatible): New.
* target-descriptions.c (arch_p): New VEC_P type.
(struct target_desc): New member compatible.
(free_target_description): Handle it.
(maint_print_c_tdesc_cmd): Likewise.
(tdesc_compatible_p): New function.
(tdesc_add_compatible): New function.
* xml-tdesc.c (tdesc_end_compatible): New function.
(target_children): Handle <compatible> element.
* arch-utils.c (choose_architecture_for_target): Accept target
description instead of BFD architecture as input. Query target
description for compatible architectures.
(gdbarch_info_fill): Update call.
* NEWS: Mention <compatible> element of target descriptions.