gencgc: Defer moving pinned pages to newspace as late as possible.

gencgc: Defer moving pinned pages to newspace as late as possible.

  * Rather than moving pinned pages to newspace immediately, defer
moving them until just before we start to scavenge (evacuate) all
of the oldpsace pages.

  * This, in theory, makes it easier to move pages to newspace if
they are mostly-live, rather than having to allocate new pages for
the data (increasing peak address-space use during GC), assuming
that we know that some page meets such criteria.

  * While we're here, commentary updates also replace an "XX I'd
rather not do this but the GC logic can't cope with not doing it"
with an actual explanation of WHY it needs to be done.  In fact,
commentary updates explain it twice, in two different locations.