[WebGPU] allow build WebGPU EP for WebAssembly (#23364)
### Description
This PR allows WebGPU EP to be built with Emscripten for WebAssembly,
Including:
- cmake build files update to support correct setup for Emscripten.
- code changes to fix build breaks for wasm
- change in Web CI pipeline to add a build-only target for wasm with
`--use_webgpu`.
