Enable support for Emscripten in CMake

[aiusepsi]

Addresses #3455

Takes the approach of detecting the use of Wasm SIMD by detecting if the requisite flags have been added to CMAKE_C_FLAGS or CMAKE_CXX_FLAGS. Adding the SIMD flags to these variables is the right way to enable Wasm SIMD globally for CMake builds, so this should do the right thing.

The use of SET(XNNPACK_ENABLE_RISCV_VECTOR OFF) is to suppress a compile error; without this RISC-specific code is compiled for Wasm; turning this option off here seemed the minimally invasive solution.

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