sim: Add a void * analogue to VPtr.

sim: Add a void * analogue to VPtr.

The default type for VPtr is now void, and the void partial
specialization of VPtr is basically just a fancy container for Addr. Its
purpose is to distinguish guest addresses from actual uint64_t-s in the
signature of simcalls so that types which are purposefully 64 bits will
stay that way, and addresses will scale to the size of pointers in the
target ABI.

Change-Id: I71e2201f5917005861ba678c6675dbcbaa0965b3
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40497
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>