intel_stub: handle missing libc_fstat{,64}

D Scott Phillips requested to merge scott-ph/shader-db:wip/2020-03/fstat into master

On Fedora 31, no longer has symbols for fstat or fstat64. I was not able to determine when this change occurred by inspecting the history of glibc or glibc.spec, but they're gone now. If dlsym() returns NULL for fstat{,64}, use __fxstat{,64} as appropriate.

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