website: Update learning-gem5 examples to explicitly set X86

It is now expected in gem5 that ISA targets are explicitly set via CPU
selection. This change makes these changes to the learning-gem5
materials.

This change: https://gem5-review.googlesource.com/c/public/gem5/+/63335
updates the scripts within the gem5 repository.

This patch also changes `DerivO3CPU` to `O3CPU`, to keep this
consistent.

Change-Id: I138153ecdc4732ea6f3ef6c11f91f2974d6cca9e
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/63336
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: Jason Lowe-Power <power.jg@gmail.com>
11 files changed