Cleanup whitespace.
Make FPU independent from DRISC.

This patch removes the dependency on DRISC in the FPU model. Now FPU
declares its own IFPUClient interface, which can be implemented by any
component. DRISC::RegisterFile implements it.

As a side effect of this change a number of header dependencies have
been removed, resulting in slightly shorter build times and
finer-grained make dependencies.