Clean up <sched.h>.

This patch switches to using the uapi constants. It also adds the missing
setns system call, fixes sched_getcpu's error behavior, and fixes the
gensyscalls script now ARM is uapi-only too.

Change-Id: I8e16b1693d6d32cd9b8499e46b5d8b0a50bc4f1d
diff --git a/libc/arch-aarch64/syscalls/setns.S b/libc/arch-aarch64/syscalls/setns.S
new file mode 100644
index 0000000..2ce2a75
--- /dev/null
+++ b/libc/arch-aarch64/syscalls/setns.S
@@ -0,0 +1,21 @@
+/* Generated by gensyscalls.py. Do not edit. */
+
+#include <private/bionic_asm.h>
+
+ENTRY(setns)
+    stp     x29, x30, [sp, #-16]!
+    mov     x29,  sp
+    str     x8,       [sp, #-16]!
+
+    mov     x8, __NR_setns
+    svc     #0
+
+    ldr     x8,       [sp], #16
+    ldp     x29, x30, [sp], #16
+
+    cmn     x0, #(MAX_ERRNO + 1)
+    cneg    x0, x0, hi
+    b.hi    __set_errno
+
+    ret
+END(setns)