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: Pull style hook management out of the main SConstruct.

Put the code which supports style hooks for mercurial and git into two
scons "tools".

Change-Id: I3ffed85a177be4f9e458fff7b1cf16a3a479914e
Reviewed-on: https://gem5-review.googlesource.com/5563
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>