By default we build SwiftShader with the LLVM backend, so the build
shouldn't fail because of missing the Subzero submodule's code. Run
git submodule update --init
To pull the submodule if you do want to build with Subzero.

Change-Id: Ib752edf891a6dd18fff8ce0dfae79ff5a1b7e011
Reviewed-on: https://swiftshader-review.googlesource.com/9088Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>