shared: use union instead of _nm_alignas() for static hash-seed

We want the the hash-seed array is alined so it can be used both as
guint, guint32, and guint64 directly. Don't use _nm_alignas() but
instead just add the fields to the union so we get proper alignment.

While at at, also let the seed argument to c_siphash_init() be aligned
to 64 integers. c_siphash_init() does not require that, but it tries to
read the seed as (unaligned) LE 64 bit integers. So, it doesn't hurt.
12 jobs for th/authchain-cleanup in 28 minutes and 24 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #293577
allowed to fail
checkpatch

00:00:58

passed #293584
t_debian:9

00:14:53

passed #293586
t_debian:sid

00:15:09

manual #293585
allowed to fail manual
t_debian:testing
passed #293578
t_fedora:28

00:23:06

passed #293579
t_fedora:29

00:28:21

passed #293580
t_fedora:30

00:22:43

manual #293581
allowed to fail manual
t_fedora:rawhide
passed #293582
t_ubuntu:16.04

00:15:21

passed #293583
t_ubuntu:18.04

00:16:12

 
  External
passed NetworkManager test verification

passed NetworkManager test verification

05:50:32