re PR ada/91995 (gnat miscompilation and bootstrap failure on m68k-linux)

re PR ada/91995 (gnat miscompilation and bootstrap failure on m68k-linux)

	PR ada/91995
	* sem_ch8.adb (Chain_Use_Clause): Remove second argument in calls
	to Defining_Entity.
	* sem_elab.adb (Find_Unit_Entity): Likewise.  Deal with N_Subunit
	here in lieu of in Defining_Entity.
	* sem_util.ads (Defining_Entity): Remove 2nd and 3th parameters.
	* sem_util.adb (Defining_Entity): Remove 2nd and 3th parameters,
	and adjust accordingly.  Deal with N_Compilation_Unit.

From-SVN: r276916