[lavapipe] VK_FUCHSIA_external_semaphore implementation

Bug: 42086544

Change-Id: I4446777f0de0748a3cf58bfeed2d8715fa323f19
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/mesa/+/991013
Reviewed-by: Josh Gargus <jjosh@google.com>
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/mesa/+/1005916
Reviewed-by: John Rosasco <rosasco@google.com>
Commit-Queue: John Rosasco <rosasco@google.com>