Add external semaphore support for Linux and Android
based on a process-shared pthread mutex + condition
variable stored in a shared memory region backed by
memfd_create().

This takes care of waiting for external semaphores
in a background thread when invoked from a Yarn fiber.

Test: dEQP-VK.api.external.semaphore.opaque_fd*

Bug: b/140421726
Change-Id: Ifa74c807d3e33914e5a37dd96650c312246c8e4f
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/35939Reviewed-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Tested-by: David Turner <digit@google.com>
Kokoro-Presubmit: David Turner <digit@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>