rts: Make markLiveObject thread-safe

Previously markLiveObject was not thread-safe, which ended up breaking during parallel GC. Unfortunately, this race generally only manifests in larger programs on with larger core counts, so it's not clear what a test should look like.

This was merged for %8.10.5 in !5127 (closed).

Fixes #19417 (closed) and perhaps #19222 (closed).