[libc] Fix compilation on 32-bit systems

This fixes the following compilation error: no known conversion from 'off_t *'
(aka 'long long *') to 'long' for 5th argument.

Since pointers are 32-bit long anyway, casting it to long shouldn't be a
problem. Tested on rv32.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D157792