When building the entire LLVM+SPIRV+libclc+... stack at the same time, libclc doesn't build spirv-mesa3d-.spv and spirv64-mesa3d-.spv

When trying to build the entire stack (LLVM+SPIRV+libclc+...) at the same time, libclc's build process doesn't see the `llvm-spirv` executable being built (it doesn't exist yet when cmake is run for libclc), and therefore disables building `spirv-mesa3d-.spv` and `spirv64-mesa3d-.spv` unless it is pointed at a pre-existing compatible copy of `llvm-spirv`.