ipc: Refactor sysvipc internal definitions

This patch refactor the internal sysvipc in two main points:

  1. Add a new __ASSUME_SYSVIPC_DEFAULT_IPC_64 to infer the __IPC_64
     value to be used along either the multiplexed __NR_ipc or wired-up
     syscall.  The defaut value assumed for __IPC_64 is also changed
     from 0x100 to 0x0, aligning with Linux generic UAPI.  The idea
     is to simplify the Linux 5.1 wire-up for sysvipc syscalls for
     some 32-bit ABIs (which expectes __IPC_64 being 0x0) and simplify
     new ports (which will no longer need to add ipc_priv.h).

  2. It also removes some duplicated internal definition used on compat
     sysvipc symbols defined at ipc_priv.h (more specifically the
     __old_ipc_perm, SEMCTL_ARG_ADDRESS, MSGRCV_ARGS, and
     SEMTIMEDOP_IPC_ARGS).  The idea is also to make it simpler to enable
     the new wire-up sysvipc syscall provided by Linux v5.1.

There is no semantic change expected on any port.  Checked with a build
against all affected ABIs.

Reviewed-by: Florian Weimer <fweimer@redhat.com>