Reactor backend: Subzero

Change-Id: Idbdc8cad8a31d60e02db59e8dd1d2bf325ee5f21
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/47748
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>