Add WASM/Emscripten compiler flags to runtime_wrapper.bzl

[s09g]

Summary:
Add pthread compiler flags (-pthread, -matomics, -mbulk-memory) for
Emscripten/WASM builds in ExecuTorch runtime wrapper.

These flags are required when linking into WASM binaries that use
-sUSE_PTHREADS=1. Without them, wasm-ld fails with:
"error: --shared-memory is disallowed by .o because it was not
compiled with 'atomics' or 'bulk-memory' features."

Reviewed By: GregoryComer

Differential Revision: D95904580

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/18025

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❌ 3 New Failures

As of commit cf8d6ed with merge base 7824373 ():

NEW FAILURES - The following jobs have failed:

• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 20a9a8140525cf121b5dec7dd0393d3eae71b2eb8470c36331368df2d3f15f38 /exec failed with exit code 1
• pull / test-samsung-quantmodels-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 71a4f6b43799c44f1f5b28c7f2a76d696fa93b30780305c35882c50375072604 /exec failed with exit code 1
• pull / test-vulkan-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t a8e2952e77868b6baa2c0b9e651612b5491ba32c62043e96f9cbf0aa180b702c /exec failed with exit code 56

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[meta-codesync]

@s09g has exported this pull request. If you are a Meta employee, you can view the originating Diff in D95904580.

[s09g]

@pytorchbot label "release notes: build"