scons: Import print_function from future

Scons on Fedora 27 imports print_function from the future[1] as a
result of which a gem5 build errors out with a syntax error. Make all
the scons scripts that use the print statement import the print_function
from future and replace the statements with print function calls.

[1] https://github.com/SCons/scons/commit/34cf3bdb1743de9a5534bfd25998d0a01297f004

Change-Id: I67b7ef978fd7567f94d3cd9a904f8a0c1af07ffb
Signed-off-by: Siddhesh Poyarekar <siddhesh.poyarekar@gmail.com>
Reviewed-on: https://gem5-review.googlesource.com/8321
Reviewed-by: Gabe Black <gabeblack@google.com>
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Maintainer: Gabe Black <gabeblack@google.com>

scons: Move python path management out of the SConstruct.

Make site_init.py manage sys.path, and the "default" tool set
PYTHONPATH on any environment that's created. The paths to add are
tracked in a common gem5_python_paths.py.

Change-Id: I3387d4394d47a2f9c83322644cfd05909c6890fa
Reviewed-on: https://gem5-review.googlesource.com/5564
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

scons: Move scons and python version checking into site_init.py.

In an effort to shrink and modularize the main scons files, this
change pulls the scons and python version checking code out of the
main file and into site_init.py which runs before the SConstruct
starts. This will be a place to put really generic code which has to
do with the very fundemental aspects of getting scons to work. Other
checks, like checks for particular tools or particular versions of
tools, will happen in other more specialized files.

Change-Id: Icd00ecadbe1141aef4dbadcf42d6ddef1f3a701f
Reviewed-on: https://gem5-review.googlesource.com/5561
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>