target/sh4: Use cmpxchg for movco when parallel_cpus

As for other targets, cmpxchg isn't quite right for ll/sc,
suffering from an ABA race, but is sufficient to implement
portable atomic operations.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Message-Id: <20170907185057.23421-2-richard.henderson@linaro.org>
[aurel32: fix whitespace]
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
