Add atomic_swap_32() and atomic_swap_64()

The atomic_swap_32() function maps to atomic_xchg(), and
the atomic_swap_64() function maps to atomic64_xchg().

Signed-off-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #377