base: Replace cppversion == version with >= version

base: Replace cppversion == version with >= version

For forward compatibility with later compilers, we should have a greater
than comparison instead of an explicit equality with a particular C++
version.

Change-Id: If848097420b9575f80134986410da3dab32567da
Signed-off-by: Bradley Wang <radwang@ucdavis.edu>
Reviewed-on: https://gem5-review.googlesource.com/11871
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>