Allows us to configure Reactor backend optimization level via
preprocessor macro. Can be set via CMake arg.

Bug: b/145685985
Change-Id: I27a80b15846a8b43be5bff0505632213852a5dc6
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/39168Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Antonio Maiorano <amaiorano@google.com>