Linux 4.18 added support for SIGSYS info in signalfd. Add the new
fields to signalfd_siginfo.

While the kernel has support for these new fields now, no libc has
shipped a release with the new signalfd fields.

Remove false support for power and mips.
Fix aarch64 definitions of nlink_t and blksize_t.

Recent change #925 added constants for IPV6 multicast, but seems
to have inadvertently left off IPV6_UNICAST_HOPS from fuchsia.

Fixes #932