(2a/n) Add WASM/Emscripten compiler flags to third-party dependencies

[s09g]

Summary:
Add pthread compiler flags (-pthread, -matomics, -mbulk-memory) for
Emscripten/WASM builds across third-party dependencies and 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."

Changes:

• xnnpack.buck.bzl (fbcode + xplat): Add WASM_EMSCRIPTEN_COMPILER_FLAGS select
• runtime_wrapper.bzl (fbcode + xplat): Add emcc compiler flags
• cpuinfo/BUCK: Add wasm-emscripten compatible_with, compiler_flags, and emscripten/init.c wrapper
• clog/BUCK: Add wasm-emscripten compiler_flags
• pthreadpool/BUCK: Add compiler_flags and WASM preprocessor_flags (condvar mode)

Reviewed By: GregoryComer

Differential Revision: D94744813

[pytorch-bot]

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❌ 3 New Failures, 1 Cancelled Job

As of commit 58c9b52 with merge base 122fdef ():

NEW FAILURES - The following jobs have failed:

• pull / android / run-emulator (gh)
  The process '/usr/bin/sh' failed with exit code 1
• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t c360894da187e9480dc2514d1547210310f572d76b5ec1ae4890c745ec7bb2ef /exec failed with exit code 1
• pull / test-samsung-quantmodels-linux / linux-job (gh)
  RuntimeError: Command docker exec -t b2f9e84779b453c1591de28a753a90cb5dd1cc49448f1c26adfa727f9c345ec7 /exec failed with exit code 1

CANCELLED JOB - The following job was cancelled. Please retry:

• pull / unittest / macos / macos-job (gh)
  ##[error]The operation was canceled.

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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 D94744813.

[s09g]

@pytorchbot label "release notes: build"

[8Keep]

Review automatically exported from Phabricator review in Meta.

[meta-codesync]

@s09g has imported this pull request. If you are a Meta employee, you can view this in D94744813.

[s09g]

@pytorchbot rerun -j "android / run-emulator"

[pytorch-bot]

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