cpu: Move FuncUnit.py, its .cc and StaticInstFlags.py above Return.
cpu: Move FuncUnit.py, its .cc and StaticInstFlags.py above Return.

These two files can be used even with the NULL ISA since they don't
depend on anything that would require a NULL ISA specific definition.

Change-Id: Ie7d83fccf39218c40bd741c76a1c6c0ac7fe2f87
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/51831
Reviewed-by: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Maintainer: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5