bigint: bug with division of a small number by a bigint

[hoshiumiarata]

It seems that the division of a small number by a bigint, where one of the operands is a negative number, is not working correctly.

For example, the following code:

-1 / (10**20)

mruby returns 1, while ruby 3.3 returns -1.

The same issue occurs with the divmod method:

-1.divmod(10**20)

mruby returns [1, 99999999999999999999], while ruby 3.3 returns [-1, 99999999999999999999].

Probably, there is a problem with the calculation of the sign of the result after rounding.

[matz]

Now we can merge #6444