Add simple arg passing to MC-JIT and support for exit() call.

Support argument passing simple, common, prototypes directly. More
complicated scenarios will require building up a stub function, which the
MC-JIT isn't set up to handle yet.

Add Intercept.cpp, which is just a copy from ExecutionEngine/JIT for now,
to handle looking looking up external symbol names. This probably more
properly belongs as part of RuntimeDyld. It'll migrate there as things
flesh out more fully.

llvm-svn: 128090