sparc64: simple microoptimizations for atomic functions

Simple microoptimizations for sparc64 atomic functions:
Save one instruction by using a delay slot.
Use %g1 instead of %g7, because %g1 is written earlier.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>