Make fpu optional

Created by: JeanRochCoulon

As FPU size is significant (114 equivalent kgates in our techno), it is critical to have the capability to remove it if it is not used by our product.

To disable the FPU, RVD and RVF are set to zero. With this configuration, the resulting RTL cannot be elaborated by the Synopsys dc_shell tool.

The proposed modification solves the issue.

Regards, Jean-Roch