Add support for the FHM extension.

This includes all forms of FMLAL, FMLAL2, FMLSL and FMLSL2.

Simulator tests are included, but the traces were generated with the simulator
itself. Later, we'll test these on hardware to verify that they are correct. In
the meantime, some basic Assembler tests are included as a sanity check.

Change-Id: If41a8c7041a9995fbe268051896a2439de5f841d