Idea: aliased heap for checking to-space invariant

[Aleksey Shipilev]

So I have been walking and muttering to myself how we cannot mprotect(PROT_READ) the collection set,
because we have to accept the fwdptr update in the same page. We used to mprotect cset for
verification, but that code basically mprotect(PROT_WRITE)-ed the page when fwdptr write had
faulted, restarted the fwdptr update, accepting everything else after that too. Thus it was became
too racy to be useful. This was the reason for us to ditch that verification part, and instead rely
on explicit ShenandoahStoreCheck machinery.

Then it hit me: the memory protection is enforced on virtual pages, not on physical pages, which
means we can use the aliased heap to accept the fwdptr stores, while normal heap cset is protected
from writes! I.e. have the normal heap WRITE|READ as usual, have the alias heap WRITE|READ as usual,
then when cset is selected WRITE-protect the cset, and watch out for failures. The fwdptr updates
from WB code should instead go via the aliased heap that is WRITE-enabled.

This gives us several advantages:
  *) We capture all bad writes mechanically, instead of hoping we covered all ShStoreCheck cases
  *) The upstream exposure in .ad and platform-specific macro-assemblers goes away
  *) Roman's work on aliased heaps is not in vain :)
  *) We don't arrive to the mess with "differently-shaped" pointers to both normal and aliased heap,
because we never leak aliased heap pointers anywhere: we just use that as the location for the
fwdptr CAS.

We can (and probably should) only enable this for verification, so we don't have any ill effects for
non-verificated modes (which would just do the same thing they do today).

Thanks,
-Aleksey