ada: Make "finally" a non-reserved keyword

This makes it possible to use the word "finally" as an identifier when
GNAT extensions are enabled. This does not make the syntax ambiguous
and the "finally" syntax extension is not affected.

gcc/ada/ChangeLog:

	* doc/gnat_rm/gnat_language_extensions.rst: Add that finally is a
	non-reserved keyword.
	* par-ch5.adb (P_Sequence_Of_Statements): handle "finally" syntax
	extension.
	* snames.ads-tmpl: Remove the concept of GNAT extension reserved
	word.
	* snames.adb-tmpl (Is_Keyword_Name): Likewise.
	* scans.adb (Initialize_Ada_Keywords): Likewise.
	* par-util.adb (Check_Future_Keyword): Likewise.
	* gnat_rm.texi: Regenerate.