[libcxx] Use integer division

In Python 3, math.floor returns int when both arguments are ints.
In Python 2, math.floor returns float. This leads to a failure
because the result of math.floor is used as an array index. While
Python 2 is on its way out, it's still used in some places so use
an integer division instead.

Differential Revision: https://reviews.llvm.org/D99520