[Ada] GNAT.Serial_Communications: simplify the Serial_Port structure

[Ada] GNAT.Serial_Communications: simplify the Serial_Port structure

2019-07-08  Dmitriy Anisimkov  <anisimko@adacore.com>

gcc/ada/

	* libgnat/g-sercom.ads
	(Serial_Port_Descriptor): New type.
	(Serial_Port): Add a comment, make it hold a
	Serial_Port_Descriptor.
	(To_Ada, To_C): New procedures.
	(Port_Data, Port_Data_Access): Remove types.
	* libgnat/g-sercom.adb (To_Ada): New stub.
	* libgnat/g-sercom__linux.adb, libgnat/g-sercom__mingw.adb:
	Update implementations accordingly.
	* s-oscons-tmplt.c: Bind Serial_Port_Descriptor to
	System.Win32.HANDLE on Windows, and to Interfaces.C.int on
	Linux. Add "Interfaces.C." prefix for other basic integer type
	bindings.
	* xoscons.adb (Output_Info): Remove the "Interfaces.C." prefix
	for subtypes generation.

From-SVN: r273225