libgo: add missing _arpcom struct to *sysinfo.go

This struct is filtered due to having a field of type _in6_addr,
but other types exported to *sysinfo.go are depending on it.

Issue golang/go#19200

Change-Id: I090d67906c0d7191c31e827c3c60adad7a4f242e
Reviewed-on: https://go-review.googlesource.com/38251
Reviewed-by: Ian Lance Taylor <iant@golang.org>
2 files changed