Enable FPU support

[Dolu1990]

There is an example of usage :

./make.py --with-fpu --cpu-count 2 --build --load --board=arty 

To generate the buildroot image, just uncomment in the buildroot defconfig :

• BR2_RISCV_ISA_CUSTOM_RVF=y
• BR2_RISCV_ISA_CUSTOM_RVD=y

That's it :)