[Ada] Fix inconsistent handling of character set control switches

[Ada] Fix inconsistent handling of character set control switches

gcc/ada/

	* csets.adb (Initialize): Refactor into CASE statement; raise
	exception on unsupported code of character set (it will be
	gently rejected earlier when scanning command line switches).
	* switch-b.adb (Scan_Binder_Switches): Refactor into a
	membership expression; add missing '9' choice; reorder as
	described by GNAT UG, section 4.3.11.
	* switch-c.adb (Scan_Front_End_Switches): Refactor into a
	membership expression and reorder as above.
	* doc/gnat_ugn/building_executable_programs_with_gnat.rst
	(gnatic): Mention '5' as an allowed value for "c".
	* gnat_ugn.texi: Regenerate.