opencl: refactor backend initilization (llama/23318)

* opencl: refactor initialization

* opencl: refactor GPU identification

* opencl: rename for consistency

* opencl: cache global mem size in dev_ctx

* opencl: adjust log level

* opencl: load argsort and flash_attn kernels in supports_op

* argsort kernel must be built for supports_op for querying the max
  workgroups
* flash_attn kernel has many variants, only load them when needed