Basically a partial clone of std::condition_variable, but works with fibers / the scheduler.

Bug: b/139010488
Change-Id: Ia89a4930f8c203b03197a4dda9ddd585ae5d8e40
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/34815Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>