Add validate_asmjs.py tool to help unit-testing whether a build output is proper asm.js or not.
Add CMake helper function em_validate_asmjs_after_build() that adds an extra post-build step to project to confirm that the build output is valid asm.js.
Use em_validate_asmjs_after_build() in CMake test.
Use SPIDERMONKEY_ENGINE from ~/.emscripten in validate_asmjs.py as the primary source for locating SpiderMonkey. If it was not set appropriately, look from PATH.
Merge pull request #1608 from juj/validate_asmjs

Validate asmjs