sysvipc: Split out linux struct shmid_ds

This will allow us to have architectures specify their own version.

Not semantic changes expected.  Checked with a build against the
all affected ABIs.

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Tested-by: Alistair Francis <alistair.francis@wdc.com>
Tested-by: Carlos O'Donell <carlos@redhat.com>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>