pulseaudio: remove BR2_ARCH_HAS_ATOMICS dependency

pulseaudio is able to either use the atomic __sync builtins from the
compiler, or to rely on libatomic_ops for atomic operations. However,
since it anyway selects json-c which requires the __sync built-ins, it
means using libatomic_ops is useless: even if you use libatomic_ops
for pulseaudio, you'd still get a link error in pulseaudio due to the
missing __sync built-in for the json-c library.

Also, since pulseaudio now inherits the BR2_TOOLCHAIN_HAS_SYNC_4 from
json-c, which matches the __sync built-in from pulseaudio, this
commit:

 - Drops the BR2_ARCH_HAS_ATOMICS dependency
 - Forces pulseaudio to not detect libatomic_ops
 - Propagates the removal of BR2_ARCH_HAS_ATOMICS dependency to
   pulseaudio's reverse dependencies.

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