Native optimizer windows work

Work on making native optimizer functional on Windows:
 - Allows configuring native optimizer location in .emscripten and in an environment variable.
 - Adds workarounds to enable Visual Studio 2012 build in addition to Visual Studio 2013.
 - Fix VS2012+VS2013 breakage on parsing "0xFF" with strtod.
 - Adds an assert() to expose a failure in simplifyOps pass.