[libc] Implement fopen, fclose, and fread on the GPU

This patch implements the `fopen`, `fclose`, and `fread` functions on
the GPU. These are pretty much re-implemented from what existed but
using the new interface. Having this subset allows us to test the
interface a bit more strenuously since we can write and read to a file.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D157622