Make tdesc_reg string fields std::string

Make tdesc_reg string fields std::string

Make the name, group and type fields of tdesc_reg std::strings.  This
way, we don't have to manually free them in ~tdesc_reg.

Doing so results in a small change in the generated tdesc.  Instead of
passing an empty string for the group parameter of tdesc_create_reg, the
two modified tdesc now pass NULL.  The end result should be the same.

gdb/ChangeLog:

	* target-descriptions.c (struct tdesc_reg) <tdesc_reg>: Change
	type of name_ parameter, adjust to std::string change.
	<name, group, type>: Change type to std::string.
	<~tdesc_reg>: Replace with default implementation.
	<operator==>: Adjust.
	(tdesc_find_register_early): Adjust.
	(tdesc_register_name): Adjust.
	(tdesc_register_type): Adjust.
	(tdesc_register_in_reggroup_p): Adjust.
	(class print_c_tdesc) <visit>: Adjust.
	(class print_c_feature) <visit>: Adjust.