add log_callback to llama_context_params for custom logging. (#2234)

* add log_callback to llama_context_params for custom logging.

* Fix macro expansion on gcc

* Add struct llama_state for global variables and move log_callback there

* Turn log level into enum and some minor changes.

* Remove model_for_logging parameter (not needed anymore)

* Convert remaining fprintf(stderr, ...) calls to use new macros.

* Fix enum and initialize g_state

* Fix log calls after merge

* Fix missing static

* Add back all the new lines in the logging strings

* Add comment for llama_log_callback and replace remaining printf calls

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Co-authored-by: grahameth <->
Co-authored-by: Helmut <[email protected]>

Author: grahameth