common.h: Avoid using unsupported load/store instructions in arm64 VMs

common.h: Avoid using unsupported load/store instructions in arm64 VMs

Using nvme show-regs within a VM on arm64 can sometimes lead to VM
termination.

To answer why this happens: one of the deficiencies of the Arm
architecture is that there exists a range of load/store instructions
that have insufficient decode information for traps taken to the
hypervisor. KVM, for example, may raise an external abort or outright
terminate the VM depending on the configuration.

This is a known problem on the kernel side, and is fixed by using
assembly MMIO accessors w/ 'safe' load/store instructions. So do
exactly that, providing arm64-specific accessors and falling back to
plain old volatile pointer accesses for other architectures.

Reported-by: William Butler <wab@google.com>
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>