use 64-bit shmid_ds layout in the generic version of bits/shm.h

this layout is more common already than the old generic, and should
become even more common in the future with new archs added and with
64-bit time_t on 32-bit archs.

the duplicate arch-specific copies are not removed yet in this commit,
so as to assist git tooling in copy/rename tracking.

arch/generic/bits/shm.h

@@ -4,11 +4,8 @@ struct shmid_ds {
 	struct ipc_perm shm_perm;
 	size_t shm_segsz;
 	time_t shm_atime;
-	int __unused1;
 	time_t shm_dtime;
-	int __unused2;
 	time_t shm_ctime;
-	int __unused3;
 	pid_t shm_cpid;
 	pid_t shm_lpid;
 	unsigned long shm_nattch;