enable printing release-build stats to stderr on exit

For evaluating memory usage when running DR on large apps, debug build is sometimes not an option, and we want to evaluate release build's leaner footprint in any case. We already have a notion of "rstats" or "release-build stats" but today they are only visible by peeking into process memory or exposing via shared memory as is done with DRstats. Especially on UNIX, it would useful to have an option to print them to stderr on exit.