Fix NaN Canoicalization Issue for SIMD Bitcasts

Fix for continued discussion on https://github.com/kripken/emscripten/pull/1757

Btw,

1) I don't expect simd.js to change rapidly, but should we continue manually pull changes into Emscripten rather than using git submodules?

2) It'll be helpful in tests/runner.py to print out the js engine that it's running the test with.

3) It'll be helpful in tests/runner.py to print a message telling the user that v8 cannot be used instead of giving a mysterious message saying no available js engines when the user specified v8 in ~/.emscripten.

Thanks!!