Verify that the CPU has enough features to run the kernel.  This may
entail enabling features on some CPUs.

By doing this in the setup code we can be guaranteed to still be able to
write to the console through the BIOS.
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>