sim: Make the Process create function use the object loader mechanism.

sim: Make the Process create function use the object loader mechanism.

This gets rid of the big mass of #if-s around headers and around the
code which creates an object file.

As a nice side bonus, this also means that in addition to supporting
multiple OS/arch combinations simultaneously, the object file loader
could support multiple ISAs simultaneously as well, since each could
load and set up its object file loaders indepedently and without the
base process classes knowledge/involvement.

Change-Id: I0a19ad06e30e9062a96d27f00b66756eb3a595ba
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/18631
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>