[GH #880/#875] Try to fix some GC thread bugs

Do not ascent into parent_interpreter with a threaded interp. (No influence)
Do not mark PMCs when threaded interp is different to the current interp
(Parrot_gc_mark_PMC_alive)
add t/stress/threads.t for GH #880 and GH #875
t/stress/threads.t fixup: add comments