Port fix to PCL from Gerd Moellmann regarding metacircles:
there was a hole in the metacircularity detection of legacy PCL,
which is filled by bypassing ordinary slot access for standard
classes in cache miss handling when doing so would lead to
another cache miss.
... *CACHE-MISS-VALUES-STACK* and *STANDARD-SLOT-LOCATIONS* (not
audited for threadsafety)