Proper implementation of a VkSemaphore using an std::mutex
and a marl::ConditionVariable. This should make the implementation
compatible with both fibers and threads at the same time.

A future CL will add platform-specific implementations to cover
external semaphores as well. Which explains why the implementation
details are hidden from VkSemaphore.hpp.

Bug: b/140421726
Change-Id: I1db55493a41db0eb60ce9181fe864253db09f4f8
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/35933Tested-by: David Turner <digit@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>