fpu: round integer-to-float conversions with RMM ties-away

[carlosqwqqwq]

fpu: round integer-to-float conversions with RMM ties-away

Fixes #264

Commit message

fpu: round integer-to-float conversions with RMM ties-away

Description

The integer-to-float conversions (fcvt.s.w[u]/l[u], fcvt.d.w[u]/l[u]) use plain host casts, which only provide RNE. Under RMM (static rm=RMM or dyn + frm=RMM) the exact-halfway cases must round away from zero; the current code returns the RNE neighbour. Add magnitude-based RMM conversion helpers in src/util/fpu_lib.h (exact for inputs that fit, otherwise one rounded significand with ties-away and explicit NX), and select them from src/cpu/riscv_fpu.c when the effective mode is RMM. fcvt.d.l also now passes the resolved dynamic mode to the existing fpu_round_i64_to_f64. This is a separate source path from the float-to-integer conversion fix (the conversion direction and helpers differ), from the arithmetic RMM synthesis, and from the FMA rounding repairs.

Validation

• Halfway-input probes under static RMM and dyn + frm=RMM match native RISC-V hardware and QEMU.
• RNE/RDN/RUP controls and the existing conversion boundary cases are unchanged.