* more llvm/opencl/spir/compute progress:
   * added stubs for compute_buffer, compute_kernel and compute_queue
   * added support to select between compiling/producing spir output and ptx output in llvm_compute
   * compiling to spir code in llvm_compute now outputs a .bc file in the first stage (generated by llvm/clang 3.5) and uses the spir-encoder (from https://github.com/KhronosGroup/SPIR-Tools/tree/master/spir-encoder/llvm_3.5_spir_encoder) to convert it to a clang 3.2 spir .bc file
 * in misc locations: don't explicitly use "= type {}" for default parameters, but rather just "= {}" (this was mostly a workaround for gcc/libstdc++ issues, the latter should generally work in a standard conforming implementation)