Building with Emscripten using Cmake incorrectly identifies XNNPACK_TARGET_PROCESSOR

When using Emscripten with Cmake to build the project, I get this in the console
```
-- Building for XNNPACK_TARGET_PROCESSOR: x86
```
It then proceeds to attempt to compile with compile options that are only compatible with x86.

It seems like the CMakeList doesn't check if the Emscripten toolchain is being used or if CMAKE_SYSTEM_NAME is equal to "Emscripten". It is using CMAKE_SYSTEM_PROCESSOR to determine XNNPACK_TARGET_PROCESSOR.

It also doesn't seem like any options are configured for Wasm despite Emscripten being listed under supported systems.