sim: Rename allocate in GuestABI to prepare.

sim: Rename allocate in GuestABI to prepare.

This method can be used for allocating resources for registers and/or
arguments, but it can also be used for generic preparation for them
as well. For instance, it can be used to detect some sort of property
of the function signature as a whole (if it's variadic for instance)
which can be stored in position and used to change ABI behavior.

Change-Id: I8a090be65dc4987e35cd115562114cd1b748155f
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24106
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>