ada: Fix up handling of ghost units [PR104027]

As reported, libgnat-12.so gets PT_GNU_STACK RWE, which means it doesn't
work in some SELinux configurations.
This is caused by the a-nbnbig.o file, which is a ghost unit and since
r12-5670 the FE emits an object file for it, but exits before compile_file
has a chance to finalize it e.g. with targetm.asm_out.file_end ()
that emits the .note.GNU-stack section on various linux targets.

Fixed by not existing but instead returning early to the caller.

2022-01-14  Andrew Pinski  <apinski@marvell.com>

	PR ada/104027
	* gnat1drv.adb (Gnat1drv): After Back_End.Gen_Or_Update_Object_File
	goto End_Of_Program.