Add the -fdebug-aux-vars flag to debug variables generated by Fortran.

gcc/fortran/ChangeLog:

	PR fortran/90207
	* invoke.texi: Document -fdebug-aux-vars.
	* lang.opt: Add -fdebug-aux-vars.
	* trans.c (MAX_PREFIX_LEN): New macro.
	(create_var_debug_raw): New function.
	(gfc_create_var_np): Call create_var_debug_raw if
	flag_debug_aux_vars is set.