Imply -msimd128 when -msse* or other similar flags are specified

Right now, Emscripten errors out if one of the SIMD flags, e.g. `-msse2`, is used without also specifying `-msimd128`.

While this works fine for Wasm-specific projects, it adds friction to building existing projects relying on SSE intrinsics. For example, CMake checks that try to use those flags, error out by default, and the only ways to fix it are either 1) modify CFLAGS / CXXFLAGS / etc. on the caller side to include `-msimd128`, which will now affect _all_ targets in CMake and not only those that used `-msse2` or 2) selectively modify CMakeFiles to include this extra flag only in places where `-msse2` is used too and only on Emscripten target.

Both those options are a bit cumbersome and I'd argue the underlying friction is unnecessary. Just like `-pthread` implies enabling atomics, bulk-memory etc., `-msse2` and friends should imply enabling `-msimd128` feature, as on their own they already are a clear signal that user intends to use SIMD instructions.