cpu: add a new instruction type 'Atomic'

cpu: add a new instruction type 'Atomic'

This patch adds a new flag named 'Atomic' to support ISA implementations
that use AtomicOpFunctor to handle atomic instructions instead of a
pair of locking load and unlocking store.

Change-Id: I1fbee6e54432396cb49dfc59ad9006b75812d115
Reviewed-on: https://gem5-review.googlesource.com/8187
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>