ggml : introduce ggml_status (#750)

ggml : introduce ggml_status (#750)

* using enum as an exit code instead of macros

* update return type from enum to unsigned int

* indentation fix

* compound update
ggml_compute_exit_code -> ggml_status
changed ggml_status from a bit-field type to simple codes
ggml_status to string cast

* ggml_status to string cast

* GGML_CALL was removed

Co-authored-by: slaren <redacted>
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Co-authored-by: slaren <redacted>
Co-authored-by: Georgi Gerganov <redacted>