Add a single Reactor Coroutine benchmark so there's something to compile.

Bug: b/148660286
Change-Id: Icbc4c44dd5c0f1ab495c3b8eddce6ea90199e987
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/40809Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>