riscv_fpu: RMM (roundTiesToAway) for the scalar OP-FP ops

[SolAstrius]

Summary

Fix RMM (round to nearest, ties to max magnitude — IEEE 754 roundTiesToAway) for the scalar arithmetic ops, addressing #204. Standalone on staging; no dependency on the other in-flight FPU work.

The host FPU has no RMM mode, so under frm == RMM the host is left in round-to-nearest-even. RNE and roundTiesToAway differ only on an exact halfway tie. The previous riscv_prepare_rmm rounded toward ±inf unconditionally — correct on ties but wrong for every inexact non-tie. This replaces it with: compute in RNE, recover the exact rounding error via the library's error-free transforms (TwoSum / TwoProduct), and step one ULP outward only on a true half-ULP tie.

What's covered

• fadd / fsub / fmul / fdiv, both .s and .d.
• fmul near the underflow boundary, where Dekker's product error itself underflows — handled with an exact widened (f32) / scaled-residual (f64) tie test.
• subnormal fdiv quotients, the one division case that can land on an exact tie.
• fsqrt needs no fixup (a square root is never an exact halfway case).
• Flag-isolated throughout: the error-free transforms do raw host arithmetic whose intermediate steps can raise spurious NV/OF, so each wrapper snapshots and restores the exception flags; the genuine flags come from the base op.

Scope / draft notes

Deliberately minimal and independent, so it can be reviewed on its own (per review feedback that RMM is too complex to evaluate bundled with the rest):

• Dynamic RMM only (frm == RMM, rm == DYN). The static ,rmm instruction suffix needs host-rounding-mode forcing and lands with the rounding-mode rework.
• OP-FP only — no FMA. The FMA-family ties-away fixup depends on the FMA rounding path; under RMM the FMA ops still round ties-to-even here, to be completed with the FMA work.

Draft until the companion FPU PRs settle (it lightly overlaps the fadd/fsub/fmul/fdiv read sites with the canonicalization/mal-box PR, so ordering may want a trivial rebase).

Touches src/cpu/riscv_fpu.c only.