Validate asmjs

Adds a new python tool tools/validate_asmjs.py that acts both as a command-line utility and an importable python function, the purpose is to check that the given build output file is valid asm.js code. Useful for unit testing suites to confirm that the build output is good asm.js as expected.

Also adds a CMake helper function so people can drop that check into the CMake builds of their own projects.