As of PR #977, gem5 has a defined M5OPS_ADDR for arm64, even if it is
constrained to certain conditions. With that change and the arm64 board
KVM support (#725), it seems like interaction with the m5 utility under
arm64 will most commonly occur in KVM -- where instruction-mode does not
work -- and thus address-mode becomes more desirable as the default.
This also makes m5's behavior in arm64 consistent with x86, the only
other architecture that supports address-mode operations.