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.