x86/UV2: Fix new UV2 hardware by using native UV2 broadcast mode

Update the use of the Broadcast Assist Unit on SGI Altix UV2 to
the use of native UV2 mode on new hardware (not the legacy mode).

UV2 native mode has a different format for a broadcast message.
We also need quick differentiaton between UV1 and UV2.

Signed-off-by: Cliff Wickman <cpw@sgi.com>
Link: http://lkml.kernel.org/r/20120116211750.GA5767@sgi.com
Cc: <stable@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>