Add an loader command on arm64 to sync the cache

On boot we don't need to perform any CPU cache management when the IDC
and DIC fields in the ctr_el0 register are set. Add a command to tell
loader to ignore these fields. This could be useful, for example, if the
hardware is misreporting the values and we are missing a quirk to enable
it.

It is not expected this will be needed, but is only intended as a
workaround to ensure the kernel can still boot.

Sponsored by: The FreeBSD Foundation