syscall: add aix to syscall_unix_test.go

syscall: add aix to syscall_unix_test.go

This file was forgotten during the port of aix/ppc64. In order to make
its tests passed, a few things were added:
- Add termios.h to zerrors
- Add AF_LOCAL = AF_UNIX as this constant doesn't exits natively on AIX
- Fix the alignment in cmsghdr structure.

TestPassFD doesn't work on AIX TL<2 because getsockname isn't working as
expected with unix socket.

Change-Id: I928705bfc78ada29e66df61fe97d8f379f8c739b
Reviewed-on: https://go-review.googlesource.com/c/go/+/171339
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>