Beginnings of MC-JIT code generation.

Proof-of-concept code that code-gens a module to an in-memory MachO object.
This will be hooked up to a run-time dynamic linker library (see: llvm-rtdyld
for similarly conceptual work for that part) which will take the compiled
object and link it together with the rest of the system, providing back to the
JIT a table of available symbols which will be used to respond to the
getPointerTo*() queries.

llvm-svn: 127916