RISC-V: enable F/D fadd test

What

Implement FADD.*, FSUB.*, FMUL.* instructions, required to activate the p-fadd tests. Additionally implements FDIV.*

Why

These could be implemented with the same pattern. FSQRT.* is left to a follow up MR, FSQRT.* required an additional dependency (see !12758 (merged))

How

rustc_afloat::Float contains the relevant impl.

Manually testing the MR

cd src/risc_v
cargo test -- parser fadd