Move SvAMAGIC flag from object to stash

Move SvAMAGIC flag from object to stash

By putting the flag on the stash, we can allow the overloaded status
of all objects of a particular class to change without having to
change the flag on every object (which would require traversing arenas
or keeping caches).

This partially fixes bug #112708, in that objects that existed before
their class had any overloading will start working after overloading
is introduced if other objects are blessed into that class.

Without blessings of other objects, they still don’t work yet.  The
fix for that is yet to come....

This was also a good excuse for deleting a comment that contained two
typos.  :-)