Updating to latest emscripten

This branch is tracking changes to update to the latest version of emscripten (currently master 6010666be9) to get the latest performance improvements, including outlining and switch optimizations.

Currently, many systems build fine, but C64 is representative of the ones that fail.  In addition, I hope to be able to revert parts of [commit 996dcce](https://github.com/jsmess/jsmess/commit/996dccefbfb666033a5a8bca535d06ac0660ee8b) with the new outlining code.