x86: Fix up silly i1586 boot message.

Trying to boot a 64-bit kernel on a 32bit Pentium 4 gets
you an amusing message along the lines of.
"you need an x86-64, but you only have an i1586"
due to the P4 being family F.  Munge it to be 686.

Signed-off-by: Dave Jones <davej@redhat.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>