* config/s390/s390.c (GP_ARG_NUM_REG, FP_ARG_NUM_REG): New defines.
(s390_function_arg, s390_gimplify_va_arg): Use them.
(s390_register_info): Save only required subset of argument
registers into save area for va_list.
(s390_va_start): Initialize only required elements of va_list struct.
(s390_build_builtin_va_list): Set va_list_[gf]pr_counter_field.