msgpack: rework BR2_ARCH_HAS_ATOMICS dependency

The msgpack source code indeed checks for the 4 bytes __sync built-in,
so this commit switches the package from a BR2_ARCH_HAS_ATOMICS
dependency to a BR2_TOOLCHAIN_HAS_SYNC_4 dependency.

python-msgpack, a reverse dependency of msgpack, is changed in the
same way.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>