mnist : use CMake to build mnist wasm example (#1269)

mnist : use CMake to build mnist wasm example (#1269)

This commit updates the mnist examples to use CMake for building the
WebAssembly (WASM) version of the MNIST example instead of the current
emcc command.

The motivation for this change is that using CMake should make it easier
to maintin with regards to when changes in ggml occur they should not
cause this example to break. Currently the emcc command is outdated and
it was not clear how to updated it which is why this change was made.

Resolves: https://github.com/ggml-org/ggml/issues/1264