[amd-spirv-backend][OpenCL] Enable OpenCL support for --offload-arch=amdgcnspirv

[lamb-j]

Currently, we don't have runtime support for SPIRV on AMDGPUs for OpenCL (only HIP). We should extend the AMDGPU OpenCL Runtime to also support SPIRV, which should be a more straightforward task than what has already been done for HIP, as SPIRV natively is more compatible with OpenCL.