Make kernel option KERNVIRTADDR optional, remove it from std.<platform>
files that can use the default value.

It used to be required that the low-order bits of KERNVIRTADDR matched
the low-order bits of the physical load address for all arm platforms.
That hasn't been a requirement for armv6 platforms since FreeBSD 10.
There is no longer any relationship between load addr and KERNVIRTADDR
except that both must be aligned to a 2 MiB boundary.

This change makes the default KERNVIRTADDR value 0xc0000000, and removes the
options from all the platforms that can use the default value. The default
is now defined in vmparam.h, and that file is now included in a few new
places that reference KERNVIRTADDR, since it may not come in via the
forced-include of opt_global.h on the compile command line.