fpu: canonicalize NaN results of fadd/fmul/fdiv and fused ops

[carlosqwqqwq]

Summary

The FP writeback for fadd, fmul, fdiv and the fused fmadd/fmsub/fnmsub/fnmadd family stores the raw host result, which keeps non-canonical NaN payloads. The fsub/fmin/fmax paths already canonicalize through the write helpers; the RISC-V specification requires the canonical NaN bit pattern for arithmetic results. Route the remaining arithmetic writebacks through the same canonicalizing writers.

Signaling-NaN NV flagging is unchanged.

Validation

• qNaN-payload, inf*0+1 and 0/0 probes match native RISC-V hardware and QEMU after the fix, with the JIT enabled and disabled.
• Every arithmetic result is the canonical NaN and fflags match the references.

Fixes #254

[LekKit]

@SolAstrius please confirm via your out-of-tree test suite and review if possible, thank you