scons: Build the marshal binary in a bare minimum environment

scons: Build the marshal binary in a bare minimum environment

This change adds an additional bare minimum environment that includes
python only and changes the marshal binary to compile using it.

Change-Id: Id5d1ee6899796d746d8dc1a004cfe4795f040c55
Signed-off-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28428
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>