[bcl] Add API diff for monotouch/monodroid profiles (#4805)

[bcl] Add API diff for monotouch/monodroid profiles (#4805)

This runs the API diff against the monotouch and monodroid
profiles in addition to net_4_x to help us catch mistakes that
only affect the mobile profiles.

To make updating the API snapshot in case of an intentional change
easier without needing to build those profiles locally I added
a way to download the diff as a .patch file. You can just download
the file from the API diff Jenkins generates and apply it locally.