Reactor backend: Subzero

Change-Id: Ie685c3ae0cfb4392bf829dbb79e677fe485a87a9
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/54168
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>