S390: Restructure s390_push_dummy_call

S390: Restructure s390_push_dummy_call

Simplify the structure of s390_push_dummy_call and its various helper
functions.  This reduces the code and makes it easier to extend.  The
new code should be functionally equivalent to the old one, except that
copies created by the caller are now always aligned on an 8-byte
boundary.

gdb/ChangeLog:

	* s390-linux-tdep.c
	(is_float_singleton): Remove function.  Move the "singleton" part
	of the logic...
	(s390_effective_inner_type): ...here.  New function.
	(is_float_like): Remove function.  Inline its logic...
	(s390_function_arg_float): ...here.
	(is_pointer_like, is_integer_like, is_struct_like): Remove
	functions.  Inline their logic...
	(s390_function_arg_integer): ...here.
	(s390_function_arg_pass_by_reference): Remove function.
	(extend_simple_arg): Remove function.
	(alignment_of): Remove function.
	(struct s390_arg_state): New structure.
	(s390_handle_arg): New function.
	(s390_push_dummy_call): Move parameter placement logic to the new
	function s390_handle_arg.  Call it for calculating the stack area
	sizes first, and again for actually writing the parameters.