[Build] WASM build of `v1.20.1` with `--use_xnnpack` fails

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### Describe the issue

Hello 👋

I want to utilize the ONNX runtime in a Web application. I successfully built the static library of ONNX runtime with SIMD and threading and linked it within my Emscripten project. The CPU execution providers works great. Now I tried to also include the XNNPACK execution provider with the `--use_xnnpack` flag to improve the performance of the inferencing.

However, the `src/amalgam` directory is missing from the XNNPACK distribution. [`xnnpack.cmake`](https://github.com/microsoft/onnxruntime/blob/ff8465eda45a5ab1259fc5f1091fd38a33b596c4/cmake/external/xnnpack.cmake#L98) says:

> \# See source lists in [_deps/googlexnnpack-src/BUILD.bazel for wasm_prod_microkernels](https://github.com/google/XNNPACK/blob/0da379fc4808f9601faef392352018c741c0f297/BUILD.bazel)

I cannot find any documentation at the XNNPACK how to manually generate these amalgam (?) microkernels. I also cannot find an example in this repository. How to build the XNNPACK execution provider for the Web?

### Urgency

_No response_

### Target platform

WASM

### Build script

```
onnxruntime/build.sh \
--build_dir ./onnxruntime-build \
--config Release \
--build_wasm_static_lib \
--enable_wasm_simd \
--enable_wasm_threads \
--skip_tests \
--disable_wasm_exception_catching \
--disable_rtti \
--use_xnnpack \
--parallel
```

### Error / output

```
CMake Error at external/xnnpack.cmake:189 (target_sources):
Cannot find source file:

./onnxruntime-build/Release/_deps/googlexnnpack-src/src/amalgam/gen/scalar.c

Tried extensions .c .C .c++ .cc .cpp .cxx .cu .mpp .m .M .mm .ixx .cppm
.ccm .cxxm .c++m .h .hh .h++ .hm .hpp .hxx .in .txx .f .F .for .f77 .f90
.f95 .f03 .hip .ispc
Call Stack (most recent call first):
external/onnxruntime_external_deps.cmake:566 (include)
CMakeLists.txt:614 (include)
```

### Visual Studio Version

_No response_

### GCC / Compiler Version

_No response_

Start with build.sh and cmake/external/xnnpack.cmake, especially the source path for src/amalgam/gen/scalar.c. Compare that expectation with XNNPACK's referenced BUILD.bazel wasm_prod_microkernels sources and reproduce the provided WASM build command. Done means the XNNPACK execution provider builds successfully for WASM with --use_xnnpack.

Written by the indexing model from the issue text.