RiscV: prepare FRegister for F/D instruction sets

Emma Turnerrequested to merge
emturner@riscv-fregister into master

What

Some housekeeping, before starting to implement F/D instruction sets.

Why

We will use softfloats to implement the instruction sets, as that ensure we have full control of rounding modes required, that rust doesn't expose to us - and the opportunity to ensure NaN canonicalisation etc.

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