[llm] Fix llama/README.md

[larryliu0820]

This PR simplifies README.md's cmake command by using preset.
This pull request simplifies the build process for the Llama model and its runner by introducing a preset configuration and removing redundant build flags. Additionally, it updates the CMakeLists.txt file to enable specific features for tokenizers.

Build process simplification:

• examples/models/llama/README.md: Replaced the detailed cmake commands with a preset configuration (--preset llm) for building executorch and removed redundant flags for the Llama runner build process. This streamlines the instructions and reduces complexity.

Tokenizer configuration updates:

• extension/llm/runner/CMakeLists.txt: Added SUPPORT_REGEX_LOOKAHEAD and PCRE2_STATIC_PIC settings to enable regex lookahead support and ensure position-independent code for tokenizers.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/11543

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❌ 2 New Failures, 4 Unrelated Failures

As of commit cf99195 with merge base c6c3616 ():

NEW FAILURES - The following jobs have failed:

• pull / unittest-editable / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 4aa489d245e50950b050b50e84b7bdb15d6cc55cc11596b0dc017a735ed33ecb /exec failed with exit code 1
• trunk / unittest-nxp-neutron / linux-job (gh)
  RuntimeError: Command docker exec -t 7451b2ef13008c0ae34956484c99a97308a2d44da395d207704caeb3b241515a /exec failed with exit code 1

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

• pull / test-models-linux (add_mul, portable, linux.2xlarge) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The process '/usr/bin/git' failed with exit code 128
• pull / test-models-linux (add, xnnpack-quantization-delegation, linux.2xlarge) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The process '/usr/bin/git' failed with exit code 128
• pull / test-models-linux-basic (vit, xnnpack-quantization-delegation, buck2, linux.2xlarge, executorch-u... / linux-job (gh) (matched linux rule in flaky-rules.json)
  The process '/usr/bin/git' failed with exit code 128

BROKEN TRUNK - The following job failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• trunk / test-apple-model / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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[github-actions]

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[guangy10]

Are the instructions tested in the CI? If all green, should we merge this PR?

[larryliu0820]

Are the instructions tested in the CI? If all green, should we merge this PR?

Yeah the same command in test_llama.sh in CI.