arch: Don't add contents to the TheISA namespace in arch/generic.

arch: Don't add contents to the TheISA namespace in arch/generic.

Instead, add what you want other ISAs to be able to use to a generic,
fixed namespace, and then let those other ISAs bring those symbols in
with "using" if they want them.

Change-Id: I15bfaf56e76ffdc3bdb603deef4ad471211f4f24
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32929
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>