supports_llvm needs to include is_mac.

Bug chromium:966543

Change-Id: Ieedf1bba3948de8c75d8bd07473fa2cb0a1e4299
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/32032Reviewed-by: Alexis Hétu <sugoi@google.com>
Presubmit-Ready: Sean Risser <srisser@google.com>
Tested-by: Sean Risser <srisser@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>