Greek (el_CY, el_GR) locales: Introduce ab_alt_mon (bug 22937).

As spotted by GNOME translation team, Greek language has the actually
visible difference between the abbreviated nominative and the abbreviated
genitive case for some month names.  Examples:

May:

abbreviated nominative: "Μάι" -> abbreviated genitive: "Μαΐ"

July:

abbreviated nominative: "Ιούν" -> abbreviated genitive: "Ιουλ"

and more month names with similar differences.

Original discussion: https://bugzilla.gnome.org/show_bug.cgi?id=793645#c21

	[BZ #22937]
	* localedata/locales/el_CY (abmon): Rename to...
	(ab_alt_mon): This.
	(abmon): Import from CLDR (abbreviated genitive case).
	* localedata/locales/el_GR (abmon): Rename to...
	(ab_alt_mon): This.
	(abmon): Import from CLDR (abbreviated genitive case).