ocl: adjusted finalization-flow of OpenCL backend

[hfp]

• Fixed case when ACC_OPENCL_MEM_DEVPTR is turned off at compile-time.
• Ensure termination message appears one time at most (cleanup).
• Introduced ACC_OPENCL_EVENT_CHAIN and ACC_OPENCL_EVENT_WAIT.
• Removed compile-time option for ACC_OPENCL_MEM_CPYSYNC.
• Removed compile-time setting (ACC_OPENCL_STREAM_NULL).
• Introduced WA-levels to distinct certain WAs.
• Ensure final cleanup (atexit).

[hfp]

For the merged PR, the precommit tests are apparently failing while it they were green locally as well as for the PR prior to merge.