* s390-tdep.c: Tweak argument-passing to match GCC bugs.

* s390-tdep.c: Tweak argument-passing to match GCC bugs.
(is_float_singleton, is_struct_like, is_float_like): New
functions, that isolate the weirdness.
(is_double_or_float, is_simple_arg, pass_by_copy_ref,
is_double_arg): Use is_struct_like and is_float_like, rather than
testing the type codes ourselves.
(s390_push_arguments): When passing args on the stack, align each
on to a four-byte boundary, regardless of what the type itself
needs.