base: Add a type for keeping track of object file loaders.

base: Add a type for keeping track of object file loaders.

This avoids having a big pile of #if-s in sim/process.cc and allows
dynamically adding new types of object file loaders which might
recognize new arch/OS combinations.

Change-Id: Ie3b9c1aa2974d30a61afc4fcc529ffd6a74d43e0
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/18583
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>