arch: Move the ISA parser into a package.

arch: Move the ISA parser into a package.

This will make splitting the parser into components easier, since it
will keep help keep everything together and organized.

Change-Id: I737641e124b6da8b1b18a49de9110c8424d8cc4f
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/35277
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>