seccomp: add ppc support

seccomp: add ppc support

This patch enables seccomp support for LXC containers running on PowerPC
architectures. It is based on the latest PowerPC support added to libseccomp, on
the working-ppc64 branch [1].

Libseccomp has been tested on ppc, ppc64 and ppc64le architectures. LXC with
seccomp support has been tested on ppc and ppc64 architectures, using the
default seccomp policy example files delivered with the LXC package.

[1] https://github.com/seccomp/libseccomp/commits/working-ppc64

v2:
- add #ifdefs in get_new_ctx to fix builds on systems not having SCMP_ARCH_PPC*
  defined

Signed-off-by: Bogdan Purcareata <bogdan.purcareata@freescale.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>