[Build] --build_wasm_static_lib with --use_xnnpack produces libonnxruntime_webassembly.a with undefined xnn_*/pthreadpool_* symbols

Describe the issue

A static WASM build with XNNPACK enabled completes successfully, but the resulting libonnxruntime_webassembly.a does not contain the XNNPACK or pthreadpool objects. Any Emscripten project that links it fails at wasm-ld with undefined xnn_* and pthreadpool_* symbols.

libonnxruntime_providers_xnnpack.a is built and bundled, and it references those symbols — but nothing in the build defines them, because the XNNPACK and pthreadpool targets are never built or linked.

Cause

cmake/onnxruntime_providers_xnnpack.cmake attaches XNNPACK and pthreadpool only via onnxruntime_add_include_to_target (include dirs only, no link dependency), so CMake never builds them. bundle_static_library() in cmake/onnxruntime_webassembly.cmake discovers archives by walking LINK_LIBRARIES, so they are silently omitted from the bundle. The non-static path's target_link_libraries(onnxruntime_webassembly PRIVATE XNNPACK) is in the else() branch of if (onnxruntime_BUILD_WEBASSEMBLY_STATIC_LIB) and never runs for static builds.

Still present on main at a6e96aa. Distinct from #23460 (configure-time failure, fixed by #24860) - this surfaces only at link time in a downstream project.

Happy to raise a PR for this.

Urgency

Blocks static WASM builds with XNNPACK for any consumer.

Target platform

WASM (Emscripten, emsdk 4.0.23; host Linux x86-64)

Build script

./build.sh --config Release --build_wasm --build_wasm_static_lib
--enable_wasm_simd --use_xnnpack --skip_tests
--disable_wasm_exception_catching --disable_rtti --parallel

Error / output

wasm-ld: error: undefined symbol: xnn_initialize
wasm-ld: error: undefined symbol: xnn_deinitialize
wasm-ld: error: undefined symbol: xnn_create_convolution2d_nhwc_f32
wasm-ld: error: undefined symbol: xnn_reshape_convolution2d_nhwc_f32
wasm-ld: error: undefined symbol: xnn_setup_convolution2d_nhwc_f32
wasm-ld: error: undefined symbol: xnn_run_operator
wasm-ld: error: undefined symbol: xnn_delete_operator
wasm-ld: error: undefined symbol: xnn_delete_weights_cache
wasm-ld: error: undefined symbol: pthreadpool_create
wasm-ld: error: undefined symbol: pthreadpool_destroy
... (~20 total, all xnn_* create/reshape/setup variants and pthreadpool_*)

Visual Studio Version

No response

GCC / Compiler Version

No response

Start by reading cmake/onnxruntime_providers_xnnpack.cmake and cmake/onnxruntime_webassembly.cmake, then reproduce the issue with the provided build.sh command using Emscripten. Trace how the static bundle discovers LINK_LIBRARIES and verify that the resulting libonnxruntime_webassembly.a supplies the reported xnn_* and pthreadpool_* symbols when linked by a downstream WASM project.