softfloat: logB(0) should raise divideByZero exception

logB(0) should raise divideByZero exception from IEEE 754-2008 spec 7.3

Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Song Gao <gaosong@loongson.cn>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220930024510.800005-4-gaosong@loongson.cn>

fpu/softfloat-parts.c.inc

@@ -1436,6 +1436,7 @@ static void partsN(log2)(FloatPartsN *a, float_status *s, const FloatFmt *fmt)
             parts_return_nan(a, s);
             return;
         case float_class_zero:
+            float_raise(float_flag_divbyzero, s);
             /* log2(0) = -inf */
             a->cls = float_class_inf;
             a->sign = 1;