Allow importing file descriptors into semaphores on linux. This can be
used to synchronize ANGLE's GL renderer with respect to vulkan
composition in chromium.

Bug: angleproject:3289

Change-Id: I04ba3bbb2e343baa000ff89c21c03ca36163a713
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1623812
Commit-Queue: Michael Spang <spang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>