Add tdesc osabi and architecture functions
    
    gdb/
    	* common/tdesc.h (tdesc_architecture_name): Add new declaration.
    	(tdesc_osabi_name): Likewise.
    	* target-descriptions.c (tdesc_architecture_name): Add new function.
    	(tdesc_osabi_name): Likewise.
    
    gdbserver/
    	* tdesc.c (tdesc_architecture_name): Add new function.
    	(tdesc_osabi_name): Likewise.
    	(tdesc_get_features_xml): Use new functions.