riscv: canonicalize malformed half operands for fclass.h

[carlosqwqqwq]

Summary

On RV64, a non-NaN-boxed half value is a canonical quiet NaN. The fclass.h translation path currently passes the raw FPR value to the classification helper, unlike the other half-precision paths. Unbox the operand before classification.

Validation

• The malformed-operand witness returns the quiet-NaN class bit after the fix; the boxed control is unchanged.
• Renode, QEMU and native RISC-V hardware controls agree.
• The patch is limited to the fclass.h translation branch.

Fixes renode/renode#958