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-mips/syscalls/setns.S b/libc/arch-mips/syscalls/setns.S
new file mode 100644
index 0000000..fd4529e
--- /dev/null
+++ b/libc/arch-mips/syscalls/setns.S
@@ -0,0 +1,23 @@
+/* Generated by gensyscalls.py. Do not edit. */
+
+#include <asm/unistd.h>
+    .text
+    .globl setns
+    .align 4
+    .ent setns
+
+setns:
+    .set noreorder
+    .cpload $t9
+    li $v0, __NR_setns
+    syscall
+    bnez $a3, 1f
+    move $a0, $v0
+    j $ra
+    nop
+1:
+    la $t9,__set_errno
+    j $t9
+    nop
+    .set reorder
+    .end setns