Now that $pcc includes an offset that corresponds to $pc, we can't install
$pcc into $c0 without resetting the offset, if borrowing $pcc as a source
of ambient authority when a system class is invoked by a sandbox in CHERI
ISAv3.  Really, we should be storing a suitable $c0 in the the object's
instance data, but this gets things working in the mean time.
Initialise the CHERI system-class code capability with a suitable offset
for its entry $pc.

When initialising the system-class data capability, use a size equal to
the full sandbox memory area, not just the pointer to it; in practice,
we don't use it in this way yet, but it may be useful in the future.