target/microblaze: swx: Use atomic_cmpxchg

target/microblaze: swx: Use atomic_cmpxchg

Use atomic_cmpxchg to implement the atomic cmpxchg sequence.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>