fix rare GC segfault coming from bug in mark-loop (alternative to 53351)

[d-netto]

Should fix https://github.com/JuliaLang/julia/issues/52757, https://github.com/JuliaLang/julia/issues/53026 and https://github.com/JuliaLang/julia/issues/53350.

Removes some of the awkwardness from https://github.com/JuliaLang/julia/pull/53351 but does require waiting for all threads to join at the end of mark-loop.