llama-bench : add JSONL (NDJSON) output mode

[akx]

This PR adds a JSONL/NDJSON output mode to llama-bench. JSONL/NDJSON can be processed line by line even if the generating process fails to e.g. print out the final ] for the regular JSON output.

$ ./llama-bench -n 16,16,16,16 -ngl 99 -o jsonl > foo.jsonl
$ duckdb -c 'select n_gen, mean(avg_ts) from "out.jsonl" group by n_gen;'
┌───────┬──────────────┐
│ n_gen │ mean(avg_ts) │
│ int64 │    double    │
├───────┼──────────────┤
│     0 │   746.241384 │
│    16 │     22.08429 │
└───────┴──────────────┘

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