RISC-V: F/D conversion instructions

Emma Turnerrequested to merge
emturner@riscv-f-cvt-fmt-int into master

What

Implement the FCVT instructions from F/D instructions sets.

Why

Allows remaining F/D tests to be enabled

Manually testing the MR

cd src/risc_v
cargo test -- parser fcvt recoding structural

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