misc,scons,util: Drop support for GCC 4

misc,scons,util: Drop support for GCC 4

Corresponding website update:
https://gem5-review.googlesource.com/c/public/gem5-website/+/33657

Issue-on: https://gem5.atlassian.net/browse/GEM5-218
Change-Id: Ia72edda6229214e2f9d548266a42a0affd49b340
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/33659
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>