Add the WebAssembly target platform

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I noticed, that WebAssembly matured about a month ago and therefore I'm proposing adding WebAssembly (wasm) as a target platform alongside linux, macosx, freebsd, openbsd, mingw, minix, haiku. It might be pretty straightforward - like with Emscripten, but of course it would be best to implement a WebAssembly bytecode backend.

Adding WebAssembly as a platform should also give us a hint which functionality required by DaoVM (and modules) might be problematic (I'm talking especially about possible issues with the standard C library and dependencies due to security enforced in WebAssembly). There is also a standalone WebAssembly interpreter wasm-interp for testing purposes.

See also https://developer.mozilla.org/en-US/docs/WebAssembly .

By the way WebAssembly is enabled by default in Firefox 52+ and Chrome 57+/latest Opera (you can also run wasm code in Firefox 47+ by enabling the javascript.options.wasm flag in about:config, or Chrome (51+) and Opera (38+) by going to chrome://flags and enabling the Experimental WebAssembly flag).