drm-shim: apply file overrides for open

loader_get_pci_driver calls os_read_file on linux to get the pci id, and os_read_file uses open instead of fopen.

This allows loader_get_pci_driver to work rather than falling back to loader_get_kernel_driver_name.

For my use case, it allows the loader to choose radeonsi_dri.so rather than amdgpu_dri.so.