scons: Adjust the version of C++ to C++14.

scons: Adjust the version of C++ to C++14.

Change-Id: I318d337fc61bca0ae40413c23ee36d59d45a79bc
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34820
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Richard Cooper <richard.cooper@arm.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>