scons: Add MARSHAL_XXFLAGS_EXTRA for the marshal object

scons: Add MARSHAL_XXFLAGS_EXTRA for the marshal object

We already provide to the user the CCFLAGS_EXTRA, LDFLAGS_EXTRA
variables to pass flags to scons when compiling/linking gem5.
Those variables are not passed to the marshal object.
We add an extra pair:

MARSHAL_CCFLAGS_EXTRA, MARSHAL_LDFLAGS_EXTRA

to add flag injection capabilities to the marshal object.

The patch is also renaming base_py_env to marshal_env.
This happens for 2 reasons:

1) At the moment the marshal compilation is the only task
making use of the base python environment.

2) Consistency with the EXTRA variable names added with this patch.
I could have named them as BASE_XXFLAGS_EXTRA, but it seems too much
generic and users might be confused by that, as they might think
the BASE_XXFLAGS_EXTRA is a subset of the XXFLAGS_EXTRA so that
setting it will affect gem5 compilation as well.

Change-Id: I3e420caa897059455ff8f35462db2b38da050e93
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/30016
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>