C compiler

A backend has been added to Clang to support the MOS instruction set. This backend may be targeted by adding the flag  to clang.

The Clang is nearly compatible with the freestanding portion of the C99 standard, with a couple caveats: [[Category:C]]
 * Although LLVM's (SingleSource) end-to-end test suite passes, we haven't finished auditing the compiler for C99 compatibility. It's likely already very compatible, but we haven't done the "full point inspection" yet.
 * No float and no double. We'll eventually ship a working IEEE 754 soft float library with the compiler for completeness' sake, but we expect low demand for this, and it'll distract from the rest of the project.
 * The (default) included printf will not be compiled with floating point support, even when we ship soft float libraries. We'll find some way to link in a different version if users elect to link the soft float routines.