S390: Vector ABI support

S390: Vector ABI support

With the S390 vector ABI, vector registers are used for passing vector
arguments and for returning a vector.  Support this ABI in inferior
function calls and when setting or retrieving a function's return
value.

gdb/ChangeLog:

	* s390-linux-tdep.c: Include "elf/s390.h" and "elf-bfd.h".
	(enum s390_vector_abi_kind): New enum.
	(struct gdbarch_tdep)<vector_abi>: New field.
	(s390_effective_inner_type): Add parameter min_size.  Stop
	unwrapping if the inner type is smaller than min_size.
	(s390_function_arg_float): Adjust call to
	s390_effective_inner_type.
	(s390_function_arg_vector): New function.
	(s390_function_arg_integer): Adjust comment.
	(struct s390_arg_state)<vr>: New field.
	(s390_handle_arg): Add parameter 'is_unnamed'.  Pass vector
	arguments according to vector ABI when appropriate.
	(s390_push_dummy_call): Initialize the argument state's field
	'vr'.  Adjust calls to s390_handle_arg.
	(s390_register_return_value): Handle vector return values.
	(s390_return_value): Apply the "register" return value convention
	to a vector when appropriate.
	(s390_gdbarch_init): Initialize tdep->vector_abi.
	* NEWS: Announce S390 vector ABI support.