fix names of ipc_perm __key/__seq elements

previously the names were exposed as key/seq with _GNU_SOURCE and
__ipc_perm_key/__ipc_perm/seq otherwise, whereas glibc always uses
__key and __seq for the names. thus, the old behavior never matched
glibc, and the new behavior always does, regardless of feature test
macros.

for now, i'm leaving the renaming here in sys/ipc.h where it's easy to
change globally for all archs, in case something turns out to be
wrong, but eventually the names could just be incorporated directly
into the bits headers for each arch and the renaming removed.

include/sys/ipc.h

@@ -13,12 +13,8 @@ extern "C" {
 
 #include <bits/alltypes.h>
 
-#ifdef _GNU_SOURCE
-#define key __ipc_perm_key
-#define seq __ipc_perm_seq
-#define __key __ipc_perm_key
-#define __seq __ipc_perm_seq
-#endif
+#define __ipc_perm_key __key
+#define __ipc_perm_seq __seq
 
 #include <bits/ipc.h>