Address review comments for the previous series

Address review comments for the previous series

I failed at git and missed adding/lost changes on the wrong branch, the
result being that I didn't incorporate fixes resulting from Yao's review
comments.  This patch fixes that.

There are two places where we should use the unique pointer typedef, and
ChangeLog entries missing.

gdb/ChangeLog:

	* target-descriptions.c (struct tdesc_feature) <registers>: Use
	tdesc_reg_up typedef.
	(struct target_desc) <features>: Use tdesc_feature_up typedef.