riscv_fpu: canonicalize NaN results and mal-boxed narrow operands

[SolAstrius]

Summary

Canonicalize NaN on the scalar F/D path, on both the result and the operand side.

What's fixed

• Arithmetic and FMA results — fadd/fsub/fmul/fdiv/fsqrt and the full FMA family (fmadd/fmsub/fnmsub/fnmadd) route their result through the canonicalizing write path, so a NaN result is delivered as the canonical quiet NaN.
• Mal-boxed narrow operands — a narrow (f32) operand that is not correctly NaN-boxed in its 64-bit register reads as the canonical NaN per the RISC-V NaN-boxing rule, across arithmetic, conversions, sign-injection, min/max, compares, classification, and the FMA family.

Validation

Together with the rounding/exception work in #239, riscv-arch-test (ACT4, Spike reference) F/D/I/M reaches 260/260.

Notes

Stacked on #239 — its nine commits appear here until it merges, after which only the two canonical-NaN commits remain. No functional change outside src/cpu/riscv_fpu.{c,h}.