Improve support for FROZEN_CACHE config setting

[sbc100]

It would be nice if emscripten didn't require any write access at all in this mode.

As of today there are couple of place where we still need write access:

1. Writing the sanity.txt file (working on fixing that: Avoid writing even sanity.txt if FROZEN_CACHE is enabled #13316
2. In debug mode we call Cache.lock (which writes a lock file) to avoid writing the temp directory from multiple emscripten calls. Instead we should probably lock the temp directory.