Detailed explanation of why this PR contains a patch_3.1.74.tgz file:

Ideally, Emscripten should be able to compile C++ code that uses webgpu, only by appending -sUSE_WEBGPU=1 in link flag. What are supposed to be done under this flag is as below:

• add a webgpu.cc implementation to source list for compiling. This file includes a few inter-op code that implemented specifically for Emscripten.
• add a few JS library, including the library_webgpu.js and a few of its dependencies.

However, the actual WebGPU support in Emscripten is still ongoing, and the existing implementation in Emscripten is fall behind. Dawn maintains a newer implementation of webgpu.cc and library_webgpu.js mentioned above, and it uses a CMake target emwebgpu_c to allow C++ library to link on. So, instead of using -sUSE_WEBGPU=1, emwebgpu_c uses -sUSE_WEBGPU=0 with a few other link flags to import the newer version of library_webgpu.js and its dependencies.

The build process will generate the dependencies of library_webgpu.js, using the script in ${EMSCRIPTEN_TOOLCHAIN}/tools/maint/gen_struct_info.py. However, from Emscripten considers this script as an internal script, so it's missing from emsdk's installation.

In this PR, I include the file gen_struct_info.py inside folder cmake/patches/emscripten. If I put the original file, our Linter will be unhappy and the build is blocked, so I need to make a tar file and use cmake to extract during the build process.