[Ada] Assert_Failure vs Assertion_Error

gcc/ada/

	* libgnat/s-assert.ads (Assert_Failure): Now a renaming of
	Assertion_Error.
	* libgnat/a-assert.ads (Assertion_Error): Now a first class
	citizen.  Remove dependency on System.Assertions.
	* gcc-interface/a-assert.ads, gcc-interface/a-assert.adb: New.
	* gcc-interface/Make-lang.in (GNAT_ADA_OBJS, GNATBIND_OBJS): Add
	a-assert.o from gcc-interface.