1.0.26.17: fix GC/SIG_STOP_FOR_GC race

1.0.26.17: fix GC/SIG_STOP_FOR_GC race

Consider this: in a PA section GC is requested: GC_PENDING,
pseudo_atomic_interrupted and gc_blocked_deferrables are set,
deferrables are blocked then pseudo_atomic_atomic is cleared, but a
SIG_STOP_FOR_GC arrives before trapping to interrupt_handle_pending.
In sig_stop_for_gc_handler, GC_PENDING is cleared but
pseudo_atomic_interrupted is not and we go on running with
pseudo_atomic_interrupted but without a pending interrupt or GC.
GC_BLOCKED_DEFERRABLES is also left at 1.

Add more checks, fix comments.