scons: Override the "default" tool to set up the default env.

scons: Override the "default" tool to set up the default env.

This imports various environment variables into the scons environment,
and sets some general properties on it. These are basically just copied
directly from the SConstruct and have the same behavior here.

gem5_tool_list will be used later on to add scons "tools" which should
be automatically added to new Environment objects.

Change-Id: Ib255955090c7b1e1cb80c703c18a9c867fcf1c9e
Reviewed-on: https://gem5-review.googlesource.com/5562
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>