Make target_desc::properties an std::vector

Make target_desc::properties an std::vector

This patch changes target_desc::properties to be a vector of property
objects.  This way, we don't need to manually free the property members
as well as the property objects themselves.

gdb/ChangeLog:

	* target-descriptions.c (property_s): Remove typedef.
	(DEF_VEC_O (property_s)): Remove.
	(struct target_desc) <properties>: Make an std::vector.
	<~target_desc>: Don't manually free properties.
	(tdesc_property): Adjust.
	(set_tdesc_property): Adjust.
	(class print_c_tdesc) <visit_pre>: Adjust.