sparc64: Normalize NMI watchdog logging and behavior.

Bring this code in line with the perf based generic NMI watchdog
in kernel/watchdog.c (which we should convert over to at some
point).

In particular, don't do anything super fancy when the watchdog
triggers, and specifically don't do a do_exit() which only makes
things worse.

Either panic(), or WARN().  The latter of which will do all of
the actions such as give us a stack backtrace.

Signed-off-by: David S. Miller <davem@davemloft.net>