libstdc++: Avoid negating a size_t [pr 94747]

libstdc++:  Avoid negating a size_t  [pr 94747]

Although the code here is well formed, it doesn't show intent well.
The reason checkers trigger on this is that it is a cause of real
bugs.  So, negate a ptrdiff_t instead.

	* libsupc++/dyncast.cc (__dynamic_cast): Cast offsetof to
	ptrdiff_t before negation, to show intent more clearly.