emscripten support in Makefile

[Zibri]

I know there are projects around to compile stockfish to wasm/js.
But I am trying myself using the stockfish official sources and I am having problems.

If I compile it with:
CXXFLAGS="-msimd128 -DNNUE_EMBEDDING_OFF" make -j build ARCH=x86-64 COMP=gcc COMPCXX=em++
the error is:
wasm-ld: error: benchmark.o: must specify -mwasm64 to process wasm64 object files

if I add -mwasm64 to the final linking, I get:
wasm-ld: error: /home/runner/Stockfish/src/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/lto/libc.a(library_pthread_stub.o): wasm32 object file can't be linked in wasm64 mode

I don't know enough to solve the problem.
But it would be nice to have em++ support in the makefile.