Unify two different runtime source map parsing methods

We currently have two different ways to process source maps at runtime, both for the purpose of symbolicating stack traces:

1. `src/source_map_support.js`, enabled via internal LOAD_SOURCE_MAP setting. Used by santizers to give useful stack traces.
2. `src/emscripten-source-map.min.js` , used by `emscripten_get_callstack` and `emscripten_log_js` and `_Unwind_Backtrace` in library.js