seccomp: enable RestrictAddressFamilies on ppc64, autodetect...

seccomp: enable RestrictAddressFamilies on ppc64, autodetect SECCOMP_RESTRICT_ADDRESS_FAMILIES_BROKEN

We expect that if socket() syscall is available, seccomp works for that
architecture.  So instead of explicitly listing all architectures where we know
it is not available, just assume it is broken if the number is not defined.
This should have the same effect, except that other architectures where it is
also broken will pass tests without further changes. (Architectures where the
filter should work, but does not work because of missing entries in
seccomp-util.c, will still fail.)

i386, s390, s390x are the exception — setting the filter fails, even though
socket() is available, so it needs to be special-cased
(https://github.com/systemd/systemd/issues/5215#issuecomment-277241488).

This remove the last define in seccomp-util.h that was only used in test-seccomp.c. Porting
the seccomp filter to new architectures should be simpler because now only two places need
to be modified.

RestrictAddressFamilies seems to work on ppc64[bl]e, so enable it (the tests pass).