modula2: Add FindIndice to library module gm2-libs/Indexing

This patch introduces the procedure function FindIndice to library
module Indexing.

gcc/m2/ChangeLog:

	* gm2-libs/Indexing.def (FindIndice): New procedure
	function.
	* gm2-libs/Indexing.mod (FindIndice): Implement new
	procedure function.

Signed-off-by: Gaius Mulley <gaiusmod2@gmail.com>