cpu-o3,tests: add page boundary access test

cpu-o3,tests: add page boundary access test

The O3 model supports unaligned accesses across page boundaries.
This changeset provides a stress test for the feature.

Other benchmarks exercise it, but their coverage is spotty. This
test does nothing else except poke bytes which straddle page
boundaries.

Change-Id: Ic20c2862bcb7af7be091030c12916ba499db10c7
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22303
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>