when using testfloat the correct format is
input output unknown value
This comment requested by palmer

I get differnt answers on Rocket and Spike.  The Spike answer matches my Intel
machine.  This fails when I compute both the single precision and double
precision answer.

Two extra NaN test cases

I believe this is entirely within the RISC-V ISA spec:

 * fmv is defined to move IEEE floats
 * All otherwise-unspeficied floating-point operations canonicalize NaNs

This test fails on Spike, but passes on Rocket.  I believe this is the source
of my current torture failures.

The new RISC-V ISA spec mandates NAN bit patterns.  This test checks to make
sure that those bit patterns come out of fsqrt(-1).  These pass on both spike
and Rocket.

An additional _DWORD_RESULT version of the FP test macros is added to
facilitate floating-point tests that compare against particular bit patterns.

Add a top-level make clean target.

VM tests only support one core, so have the other cores hammer on the
memory system to attempt to catch simple coherence regressions.

Better coverage of mul high instructions