Add an loader command on arm64 to sync the cache

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