Add a fixed_size_mode_pod class

Add a fixed_size_mode_pod class

This patch adds a POD version of fixed_size_mode.  The only current use
is for storing the __builtin_apply and __builtin_result register modes,
which were made fixed_size_modes by the previous patch.

2018-01-03  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* coretypes.h (fixed_size_mode): Declare.
	(fixed_size_mode_pod): New typedef.
	* builtins.h (target_builtins::x_apply_args_mode)
	(target_builtins::x_apply_result_mode): Change type to
	fixed_size_mode_pod.
	* builtins.c (apply_args_size, apply_result_size, result_vector)
	(expand_builtin_apply_args_1, expand_builtin_apply)
	(expand_builtin_return): Update accordingly.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@256193 138bc75d-0d04-0410-961f-82ee72b054a4