Currently Windows and Linux already use the Subzero backend, while
MacOS still uses the LLVM backend. This cl switches MacOS to using
the Subzero backend.

Change-Id: Ibc20add93c5f413271832fd1dd7fe7ae00cda9f4
Reviewed-on: https://swiftshader-review.googlesource.com/11213Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>