Partially unimplemented instruction intrinsic

[pgoodman]

In some cases it might be valuable to have a partially (un)implemented instruction. For example, some complex FPU instructions like FPATAN have two real components:

1. The mechanics of how the FPU stack is modified, and
2. The actual algorithm (e.g. arctangent).

The former should be implemented in the partial instruction, as it is arch-specific and depends on no special features. The latter should be stubbed out in some way. McSema1 currently does someline like this by using LLVM intrinsics. This is probably be the most sensible approach.