scons: Marshal Python sources using the same Python as gem5

We currently use the Python version used by scons to marshal Python
code. This doesn't work when building gem5 with Python 3 support since
scons typically runs in Python 2.7. Add a custom marshal helper that
links with the same library as gem5 to generate byte code that is
guaranteed to work in gem5's Python interpreter.

Change-Id: I665b0f2078726d4c055d74a3e668a580fc613b59
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/16422
Reviewed-by: Gabe Black <gabeblack@google.com>