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 <d3e8afea784ab4a50a2dbf9149d2eaa8478e1cbc@gmail.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Bobby R. Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>

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