cpu-o3: bugfix for partial faults in x86

cpu-o3: bugfix for partial faults in x86

The c58cb8c9 changeset broke some code related to checking
consistency model guarantees (found in X86 benchmarks).

This changeset adds some documentation to the code and obviates
the problem.

Change-Id: Ied9c6b0b1d237538efe4beb2f97ef76248ce2746
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22283
Maintainer: Brandon Potter <Brandon.Potter@amd.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Brandon Potter <Brandon.Potter@amd.com>