opencl: Add support for multiple devices (llama/12622)

opencl: Add support for multiple devices (llama/12622)

* opencl: Add support for multiple devices

... but limited to one platform. A platform with a GPU will be preferred.

Additionally:

* Filter out devices that lack capabilities needed by the backend
  implementation (half support, OpenCL 2.0+, etc).

* Make ggml_backend_opencl_reg() thread-safe.

* fixup: fix an error in sync_with_other_backends

... when there is only one OpenCL device available.