Memory init file generation is ignored in wasm backend

[juj]

emcc tests\hello_world.c --memory-init-file 1 -s USE_PTHREADS=1 -s WASM=1 -o a.html

generated a file a.html.mem in asm.js backend, but does not generate the memory initializer file in wasm backend, the directive --memory-init-file 1 is getting silently ignored.

[kripken]

We should document this better, yeah. With wasm we never use a mem init file, as even with pthreads we have passive segments that let us embed them in the binary.

For wasm2js though we do obey that flag.

[tlively]

Maybe we should print a warning about the unused option?

[kripken]

Yes, docs + warning sounds good.

[stale]

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