Fix using keywords as identifiers to prevent warnings during build

Fix using keywords as identifiers to prevent warnings during build

m2pim/DynamicStrings.mod:1358:27: note: In procedure ‘Slice’: the symbol
name ‘end’ is legal as an identifier, however as such it might cause
confusion and is considered bad programming practice
 1358 |    start, end, o: INTEGER ;

m2pim/DynamicStrings.mod:1358:27: note: either the identifier has the
same name as a keyword or alternatively a keyword has the wrong case
(‘END’ and ‘end’).

gcc/m2/ChangeLog:

	* gm2-libs/DynamicStrings.mod (Slice): Rename end to stop.

Signed-off-by: Wilken Gottwalt <wilken.gottwalt@posteo.net>