llama-bench : add --no-warmup flag (#14224) (#14270)

llama-bench : add --no-warmup flag (#14224) (#14270)

Add no_warmup parameter to cmd_params struct and command-line parsing to allow users to skip warmup runs before benchmarking.

- Add no_warmup boolean field to cmd_params struct

- Add --no-warmup command-line argument parsing

- Add help text documentation for the new flag

- Wrap existing warmup logic in conditional check

- Maintain full backward compatibility (warmup enabled by default)

Addresses #14224