Lth arm port

This is Lars' port to target ARM.  Along with various fixes I added to get it working on my ARM Chromebook.  (I believe Lars had been testing his port by cross-compiling and copying the build products to a phone or other ARM device.)

For more info, see especially doc/HOWTO-FENCE, which in addition to documentation for the new high-level Fence platform, also has a number of notes from Lars on the state of his port (current bugs, etc).  Maybe we'll try to factor that info out into other documents or github tickets, who knows.