Add example for building executor_runner in Wasm

[Conarnar]

Summary

Since we want to have ExecuTorch on web builds, we first need to be able to compile it to Wasm. An example was added that gives instructions on how to build executor_runner with Emscripten, along with some modifications to CMakeLists.txt while Emscripten is used while building.

Test plan

The test script can be run with bash examples/wasm/test_build_wasm.sh from the root directory. This attempts to build executor_runner and run it on the add_mul and mv2 models.

Added a CI test-build-wasm-linux to run this script.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/12303

• 📄 Preview Python docs built from this PR

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

❌ 4 New Failures, 4 Cancelled Jobs, 1 Unrelated Failure

As of commit a21942d with merge base 6669637 ():

NEW FAILURES - The following jobs have failed:

• Lint / android-java-format / linux-job (gh)
  RuntimeError: Command docker exec -t fe09fb4efe33df28583ff4d90301c061666af032168fb8bd0b1b4f106b7413c9 /exec failed with exit code 1
• pull / android / run-emulator (gh)
  The process '/usr/bin/sh' failed with exit code 255
• pull / test-eval_llama-mmlu-linux / linux-job (gh)
  RuntimeError: Dataset scripts are no longer supported, but found mmlu_no_train.py
• pull / unittest / macos / macos-job (gh)
  backends/xnnpack/test/ops/test_conv1d.py::TestConv1d::test_qs8_conv1d_batchnorm_seq

CANCELLED JOBS - The following jobs were cancelled. Please retry:

• pull / test-models-linux (mobilebert, portable, linux.2xlarge) / linux-job (gh)
  ##[error]The operation was canceled.
• pull / test-models-linux (mobilebert, xnnpack-quantization-delegation, linux.2xlarge) / linux-job (gh)
  ##[error]The operation was canceled.
• trunk / test-models-linux-aarch64 (mobilebert, portable, linux.arm64.2xlarge) / linux-job (gh)
  ##[error]The operation was canceled.
• trunk / test-models-linux-aarch64 (mobilebert, xnnpack-quantization-delegation, linux.arm64.2xlarge) / linux-job (gh)
  ##[error]The operation was canceled.

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-qnn-model (fp32, mb) / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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[lucylq]

Thanks for adding the CI test, and all the debugging to get it working! Great job.