meson: add libatomic dependency

Add conditional dependency on GCC's libatomic in order to be able to
use the __atomic_* functions instead of the older __sync_* ones.  The
libatomic library is only needed when there aren't any native support
on the current architecture, so a linker test is used for this
purpose.  This makes atomic operations available on a wider number of
architectures including MIPS.

Signed-off-by: Guillaume Tucker <guillaume.tucker@collabora.com>
Reviewed-by: Simon Ser <simon.ser@intel.com>