Make target_desc::features an std::vector

Make target_desc::features an std::vector

This patch makes target_desc to be a vector of unique_ptr to
tdesc_feature objects.  This way, we don't have to manually free the
features and the vector in the target_desc destructor.

gdb/ChangeLog:

	* target-descriptions.c (tdesc_feature_p): Remove typedef.
	(DEF_VEC_P (tdesc_feature_p)): Remove.
	(struct target_desc) <features>: Change type to std::vector.
	<~target_desc>: Replace with default implementation.
	<accept>: Adjust.
	<operator==>: Adjust.
	(tdesc_has_registers): Adjust.
	(tdesc_find_feature): Adjust.
	(tdesc_use_registers): Adjust.
	(tdesc_create_feature): Adjust.