Commonise tdesc_feature and makes use of it in gdbserver tdesc

Commonise tdesc_feature and makes use of it in gdbserver tdesc

gdb/
	* common/tdesc.c (tdesc_feature::accept): Move to here.
	(tdesc_feature::operator==): Likewise.
	(tdesc_create_reg): Likewise.
	* common/tdesc.h (tdesc_type_kind): Likewise.
	(struct tdesc_type): Likewise.
	(struct tdesc_feature): Likewise.
	* regformats/regdat.sh: Create a feature.
	* target-descriptions.c (tdesc_type_kind): Move from here.
	(tdesc_type): Likewise.
	(tdesc_type_up): Likewise.
	(tdesc_feature): Likewise.
	(tdesc_create_reg): Likewise.

gdbserver/
	* tdesc.c (~target_desc): Remove implictly deleted items.
	(init_target_desc): Iterate all features.
	(tdesc_get_features_xml): Use vector.
	(tdesc_create_feature): Create feature.
	* tdesc.h (tdesc_feature) Remove
	(target_desc): Add features.