remove gratuitously-different arch-specific bits/ipc.h files

these differ from generic only in using endian-matched padding with a
short __ipc_perm_seq field in place of the int field in generic. this
is not a documented public interface anyway, and the original intent
was to use int here. some ports just inadvertently slipped in the
kernel short+padding form.

arch/aarch64/bits/ipc.h

@@ -1,12 +0,0 @@
-struct ipc_perm {
-	key_t __ipc_perm_key;
-	uid_t uid;
-	gid_t gid;
-	uid_t cuid;
-	gid_t cgid;
-	mode_t mode;
-	unsigned short __ipc_perm_seq;
-
-	unsigned long __pad1;
-	unsigned long __pad2;
-};

arch/riscv64/bits/ipc.h

@@ -1,12 +0,0 @@
-struct ipc_perm {
-	key_t __ipc_perm_key;
-	uid_t uid;
-	gid_t gid;
-	uid_t cuid;
-	gid_t cgid;
-	mode_t mode;
-	unsigned short __ipc_perm_seq;
-
-	unsigned long __pad1;
-	unsigned long __pad2;
-};

arch/s390x/bits/ipc.h

@@ -1,12 +0,0 @@
-struct ipc_perm {
-	key_t __ipc_perm_key;
-	uid_t uid;
-	gid_t gid;
-	uid_t cuid;
-	gid_t cgid;
-	mode_t mode;
-	unsigned short __pad1;
-	unsigned short __ipc_perm_seq;
-	unsigned long __pad2;
-	unsigned long __pad3;
-};