This enables building with either LLVM or Subzero as the Reactor JIT
compiler back-end, on both Linux and Windows.

Change-Id: I4c052d224cd1302b7de412df58b07ee275ba924f
Reviewed-on: https://swiftshader-review.googlesource.com/8375Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>