[Semaphore] make default constructor more explicit

The CancelFutureTest fails for libc++ because it uses two emper::Semaphores to coordinate the involved fibers.

Using gcc and stdlibc++ the default constructor initialized the semaphore count to 0.

The default constructor using clang and libc++ does not initialize the semaphore count member and thus causing the CancelFutureTest to misbehave.