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-arm/syscalls/setns.S b/libc/arch-arm/syscalls/setns.S
new file mode 100644
index 0000000..7afbff0
--- /dev/null
+++ b/libc/arch-arm/syscalls/setns.S
@@ -0,0 +1,14 @@
+/* Generated by gensyscalls.py. Do not edit. */
+
+#include <private/bionic_asm.h>
+
+ENTRY(setns)
+    mov     ip, r7
+    ldr     r7, =__NR_setns
+    swi     #0
+    mov     r7, ip
+    cmn     r0, #(MAX_ERRNO + 1)
+    bxls    lr
+    neg     r0, r0
+    b       __set_errno
+END(setns)