Improve the code generation for the Latin1 codec

This change does not modify the actual algorithm implemented. It only
updates the source code so that the code generation is more optimal:
 - change only one variable per loop (the "offset" variable)
 - unroll the tail expansion of the last 15 characters

The Neon code for the toLatin1 codec most likely benefits from the
unrolling of the tail too, but I can't verify that I haven't broken
anything.

Change-Id: I8a92fd3c1aa700e6f8b0c8ebdb1978ade394757f
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>