SystemInformation: Change GetRealAddress to return a size_t

The return value is not a real pointer, but an integer representing the
offset of the symbol within the loaded binary/library (possibly
including a load base). On CHERI, and thus Arm's Morello prototype,
pointers are not just integers but are unforgeable hardware capabilities
that contain additional metadata (including bounds and permissions) to
provide fine-grained spatial safety. Casting a plain integer (in this
case, a ptrdiff_t) to a void * gives a warning that we normally make
fatal, as due to the unforgeability of capabilities it will not be a
valid pointer, trapping on dereference. Whilst there are ways to squash
this warning (typically by casting via (u)intptr_t), they are not
appropriate here, since the return value does not need to be a void *,
and are normally only needed for reusing void *-taking interfaces with a
plain integer (e.g. pthread_create's void * argument). Thus change the
return type to be size_t to represent that this is the non-negative
integer offset of the address from the base address.

Note that, other than the dummy call in the constructor that throws away
the result, this member function is unused.