strongswan: needs __atomic operations

strongswan uses the __atomic_*() intrinsics, so we make it depend on
BR2_TOOLCHAIN_HAS_ATOMIC and link with libatomic when available. This
allows to fix the build on SPARC, therefore fixing:

  http://autobuild.buildroot.org/results/7e090237801874fb889c76f84863dc4c2ca20450/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>