ada: Rework CUDA host-side invocation of device-side elaboration code

ada: Rework CUDA host-side invocation of device-side elaboration code

When the binder is invoked with a "-d_c" switch, add an argument to that
switch which is the library name on the device side; so "-d_c" becomes
"-d_c=some_library_name". This does not effect the case where "-d_c" is
specified as a switch for compilation (as opposed to binding). Use this
new piece of information in the code generated by the binder to invoke
elaboration code on the device side from the host side.

gcc/ada/

	* opt.ads: Declare new string pointer variable, CUDA_Device_Library_Name.
	Modify comments for existing Boolean variable Enable_CUDA_Device_Expansion.
	* switch-b.adb: When "-d_c" switch is encountered, check that the next
	character is an "'='; use the remaining characters to initialize
	Opt.CUDA_Device_Library_Name.
	* bindgen.adb: Remove (for now) most support for host-side invocation of
	device-side finalization. Make use of the new CUDA_Device_Library_Name
	in determining the string used to refer (on the host side) to the
	device-side initialization procedure. Declare the placeholder routine
	that is named in the CUDA_Execute pragma (and the CUDA_Register_Function
	call) as an exported null procedure, rather than as an imported procedure.
	It is not clear whether it is really necessary to specify the link-name
	for this should-never-be-called subprogram on the host side, but for now it
	shouldn't hurt to do so.