Turn older #7165 prototype into an experimental feature. It can be
enabled by adding `--optimize=experimental-xforms-product-type` to the
**Additional mtouch arguments** of the project.
ref: https://github.com/xamarin/xamarin-macios/pull/7165
This allows mtouch to give better error message when something unexpected
occurs in the linker pipeline (at least for the sub-steps).
Practically it means fewer, contextless MT2001 errors. The replacements
error code are more precise, e.g.
* what was being done;
* what was being processed
and helps both diagnosing and, possibly, gives clues for workarounds
Sebastien Pouliot
Alexander Köplinger
Rolf Bjarne Kvinge
