arch,sim: Use VPtr<> instead of Addr in system call signatures.

arch,sim: Use VPtr<> instead of Addr in system call signatures.

This tells the GuestABI mechanism that these are guest pointers and not
uint64_ts, and that they should be treated as 32 bit or 64 bit values
depending on the size of pointers in the target ABI.

Change-Id: Ia9b5447848c52668a975d8b07b11ad457e756b13
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40498
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>