target/riscv: Fix fcvt.s.bf16 NaN box checking

fcvt.s.bf16 uses the FP16 check_nanbox_h() which returns an FP16
quiet NaN. Add check_nanbox_bf16() which returns a BF16 quiet NaN.

Signed-off-by: Anton Blanchard <antonb@tenstorrent.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250501114253.594887-1-antonb@tenstorrent.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
2 files changed