Modify all FreeBSD bootloaders on PowerPC AIM (Book-S) systems to pass a
magic number to the kernel in r7 rather than the (currently unused and
irrelevant) width of the metadata pointer, which I believe was intended
for a never-used approach to the 64-bit port. This enables the kernel,
in a future commit, to switch on the cookie to distinguish a real
metadata pointer from loader(8) from garbage left in r6 by some other
boot loader.

MFC after: 3 weeks