scons,python: Always generate default create() methods.

We were originally generating default create() methods along side the
pybind definitions, but unfortunately those are only included when
python support is included. Since the SimObject Param structs are
unconditionally provided even if the thing calling their create()
methods is not, we need to also unconditionally provide the default
create() definitions. We do that by putting them in their own new .cc
files.

Change-Id: I29d1573d578794b3fe7ec2bc16ef5c8c58e56d0e
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/42589
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Earl Ou <shunhsingou@google.com>
2 files changed