[Ada] Refactor repeated complains about missing -gnat2005 switch

[Ada] Refactor repeated complains about missing -gnat2005 switch

gcc/ada/

	* errout.ads (Error_Msg_Ada_2005_Extension): New routine (spec).
	* errout.adb (Error_Msg_Ada_2005_Extension): New routine (body).
	* par-ch10.adb: Reuse new routine; correct casing for "LIMITED
	WITH".
	* par-ch11.adb: Likewise.
	* par-ch12.adb: Likewise.
	* par-ch3.adb: Likewise.
	* par-ch4.adb: Likewise; replace "box" with "<>".
	* par-ch6.adb: Likewise.
	* par-ch9.adb: Likewise; correct casing for "THEN ABORT".