[Ada] Add push/pop capability in Output

[Ada] Add push/pop capability in Output

Add the capability to use the Write_* procedures in an environment where
you want to write debugging info but still use them to write to other
files (such a C source files).

gcc/ada/

	* output.ads (Push_Output, Pop_Output): New procedures.
	* output.adb (FD_Array, FD_Stack, FD_Stack_Idx): New type and vars.
	(Push_Output, Pop_Output): New procedures.