refresh compiler switches

* Add 4.2 trunk snapshots (with frame pointer variant)
* Refresh 4.1 trunk snapshots to point to the new 4.01 branch
* Remove obsolete Mirage compiler switches are are not needed since Mirage 0.9.1+
* Remove old experimental 4.1 branches (short-paths, endian) that have been since merged into mainline 4.1.

closes #798