Add environ files for using /opt/mono-sil/bin/mono

The developer/user needs to set MONO_ENVIRON to point to the environ
file, possibly in the ~/.bashrc file, or possibly in custom scripts
to run mono and xbuild.  It also needs to be set explicitly in
MonoDevelop as a project runtime option environment variable.

Change-Id: Ic40cdf0aad90f7021210dd71437a4edbaf6f345c
Merge github/develop into develop