Add support for RISC-V FPU ops

[bradenkell]

Add support for getting/setting FPU state on RISC-V. Disabling denormals is not supported by RISC-V and is left as a no-op here.

[Maratyszcza]

As suppressing subnormals on RISC-V is not supported, there's no need to support saving/restoring FPU state either, and this functionality is only used in combination with suppressing subnormals. The suggested change would result in needless saving and restoring of FPU state without any other effects.