x86: nmi - add sensible names to nmi_watchdog boot param

Impact: introduce nmi_watchdog=lapic and nmi_watchdog=ioapic aliases

Add sensible names as "lapic" and "ioapic" to
nmi_watchdog boot parameter. Sometimes it is not
that easy to recall what exactly nmi_watchdog=1
does mean so we allow the using of symbolic names here.

Old numeric values remain valid.

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>