Use jemalloc in standalone and add stats.

[jsdt]

Description of Changes

This uses jemalloc in standalone instead of mimalloc, unless the target env is msvc. The main reason is that jemalloc exposes better stats, and has a way to do heap profiling. This PR doesn't set up profiling, but it does add stats on jemalloc's memory usage.

Expected complexity level and risk

2. The risk here is that there could be an issue on some platforms.

Testing

This works on mac and at least some linux platforms.