sim: Retrofit the VPtr type.

sim: Retrofit the VPtr type.

Rename it to be ProxyPtr and ConstProxyPtr, merge it with the
functionality of BufferArg and TypedBufferArg, etc., as described in
this design doc.

https://docs.google.com/document/d/1BYHBJcf7dB2Z25zAZ9snbeRKfstK9uERYH_3h66w_tc/

Change-Id: I2fddde20cc0ece257685bc50bd3419a4e9a00145
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29400
Reviewed-by: Ciro Santilli <ciro.santilli@arm.com>
Reviewed-by: Matthew Poremba <matthew.poremba@amd.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>