Build Subzero from the subtree directory.

Build Subzero from the subtree directory.

The submodule is at /third-party/pnacl-subzero
The subtree is at /third-party/subzero

Bug swiftshader:47
Bug b/37478805

Change-Id: I2da86151f111448e962b6b24c085a2f288de18b4
Reviewed-on: https://swiftshader-review.googlesource.com/9608
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Corentin Wallez <cwallez@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>