[ExecuTorch][WebGPU] Select emdawnwebgpu for Emscripten builds

[JCNTH]

Stack from ghstack (oldest at bottom):

• [ExecuTorch][WebGPU] Add Qwen3 K16 attention and fp16 KV cache #21136
• [ExecuTorch][WebGPU] Add HuggingFace rotate-half RoPE #21135
• [ExecuTorch][WebGPU] Repair dynamic SDPA routing, add attestation #21134
• -> [ExecuTorch][WebGPU] Select emdawnwebgpu for Emscripten builds #21133
• [ExecuTorch][WebGPU] Add Llama K16 online causal attention #21132
• [ExecuTorch][WebGPU] Replace broad QKV fusion with BK64 kernel #21131
• [ExecuTorch][WebGPU] Route exact prefill shapes to the BK64 GEMM #21130
• [ExecuTorch][WebGPU] Fuse the SwiGLU elementwise chain #21129
• [ExecuTorch][WebGPU] Coordinate dynamic dispatch routes #21128
• [ExecuTorch][WebGPU] Add certified scoped output suppression #21127

Building the WebGPU backend for the browser previously required a manual
source-tree patch to pick the right WebGPU implementation. This makes the CMake
configuration select the emdawnwebgpu port for Emscripten builds while native
builds keep Dawn and their platform libraries, so the production kernel stack
builds for the browser unpatched. Build-system integration only; no model or
kernel behavior changes.

Key changes:

• CMakeLists.txt: choose emdawnwebgpu under Emscripten, Dawn otherwise.
• test/test_cmake_configuration.py: contract test asserting the selection.
  @exported-using-ghexport

Differential Revision: D113171750

Differential Revision: D113171750

[pytorch-bot]

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