[libc] Support fopen / fclose on the GPU

This patch adds the necessary support for the fopen and fclose functions
to work on the GPU via RPC. I added a new test that enables testing this
with the minimal features we have on the GPU. I will update it once we
have `fread` and `fwrite` to actually check the outputted strings. For
now I just relied on checking manually via the outpuot temp file.

Reviewed By: JonChesterfield, sivachandra

Differential Revision: https://reviews.llvm.org/D154519