jack2: use the proper BR2_TOOLCHAIN_HAS_SYNC_x symbol

Move away from BR2_ARCH_HAS_ATOMICS and use BR2_TOOLCHAIN_HAS_SYNC_4
instead for the jack2 package and its reverse dependency.

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