x86: add unknown_nmi_panic kernel parameter

It's not possible to enable the unknown_nmi_panic sysctl option
until init is run. It's useful to be able to panic the kernel
during boot too, this adds a parameter to enable this option.

Signed-off-by: Simon Arlott <simon@fire.lp0.eu>
Signed-off-by: Ingo Molnar <mingo@elte.hu>