Subzero does not support aarch64, so we should not generate Reactor unit
test targets for Subzero on Fuchsia ARM64.

Bug: b/173404759
Change-Id: Id6da8b3b4cded8a70855996ee020f55ec1d984e3
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/50588
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>