[SPARC64]: Fill dead cycles on trap entry with real work.

As we save trap state onto the stack, the store buffer fills up
mid-way through and we stall for several cycles as the store buffer
trickles out to the L2 cache.  Meanwhile we can do some privileged
register reads and other calculations, essentially for free.

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