Make target_desc::compatible an std::vector

Make target_desc::compatible an std::vector

This patch changes target_desc::compatible to be a vector of
bfd_arch_info *.  This way, we don't need to manually free the vector in
the target_desc destructor.

gdb/ChangeLog:

	* target-descriptions.c (arch_p): Remove typedef.
	(DEF_VEC_P (arch_p)): Remove.
	(struct target_desc) <compatible>: Change type to std::vector.
	<~target_desc>: Don't manually free compatible.
	(tdesc_compatible_p): Adjust.
	(tdesc_add_compatible): Adjust.
	(class print_c_tdesc) <visit_pre>: Adjust.