gdb: (de-)allocate target_desc_info with new/delete

In preparation for using non-POD types in the struct.

gdb/ChangeLog:

	* target-descriptions.c (struct target_desc_info): Initialize
	fields.
	(get_tdesc_info): Use new.
	(target_desc_info_free): Use delete.

Change-Id: I10fdaeeae7cdbd7930ae7adeeb13f7f363c67c7a