Make tdesc_feature::name an std::string

Make tdesc_feature::name an std::string

... so we don't have to manually free it in ~tdesc_feature.

gdb/ChangeLog:

	* target-descriptions.c (tdesc_feature) <name>: Change type to
	std::string.
	<~tdesc_feature>: Don't manually free name.
	<operator==>: Adjust.
	(tdesc_find_feature): Adjust.
	(tdesc_feature_name): Adjust.
	(class print_c_tdesc) <visit_pre>: Adjust.
	(class print_c_feature) <visit_pre>: Adjust.