Add preliminary support for 128-bit integer types

Add preliminary support for 128-bit integer types

This is only the gigi part, in preparation for the bulk of the
implementation.

gcc/ada/ChangeLog:
	* fe.h: Fix pilot error in previous change.
	* gcc-interface/gigi.h (enum standard_datatypes): Add ADT_mulv128_decl.
	(mulv128_decl): New macro.
	(get_target_long_long_long_size): Declare.
	* gcc-interface/decl.c (gnat_to_gnu_entity): Use a maximum size of
	128 bits for discrete types if Enable_128bit_Types is true.
	* gcc-interface/targtyps.c: Include target.h.
	(get_target_long_long_long_size): New function.
	* gcc-interface/trans.c (gigi): Initialize mulv128_decl if need be.
	(build_binary_op_trapv): Call it for 128-bit multiplication.
	* gcc-interface/utils.c (make_type_from_size): Enforce a maximum
	size of 128 bits if Enable_128bit_Types is true.