Simplify to a single implementation across all platforms.
Document the class and members.
Also add:
• Constructor with event type and initial state
• clear() method.
• wait() method with timeout.
• operator bool().

Bug: b/133127573
Change-Id: I612dc8a1011aaa2cfa50f40c59849323348b3054
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/31679
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Ben Clayton <bclayton@google.com>