sparc32: Kill btfixup for xchg()'s 'swap' instruction.

We always have this instruction available, so no need to use
btfixup for it any more.

This also eradicates the whole of atomic_32.S and thus the
__atomic_begin and __atomic_end symbols completely.

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