Make tdesc_feature::registers an std::vector

This patch makes tdesc_feature::registers an std::vector of unique_ptr
to tdesc_reg.  This way, we don't have to manually free the tdesc_reg
objects and the vector in the tdesc_feature destructor.

gdb/ChangeLog:

	* target-descriptions.c (tdesc_reg_p): Remove typedef.
	(DEF_VEC_P (tdesc_reg_p)): Remove.
	(struct tdesc_feature) <registers>: Change type to std::vector.
	<~tdesc_feature>: Don't manually free registers.
	<accept>: Adjust.
	<operator==>: Adjust.
	(tdesc_has_registers): Adjust.
	(tdesc_find_register_early): Adjust.
	(tdesc_use_registers): Adjust.
	(tdesc_create_reg): Adjust.