x86_64: add new __NR_arch_prctl syscall

This is used to set/get TLS on x86_64. There's no public declaration
of this because it's not meant to be used outside the C library, like
glibc (though we don't currently have any visibility controls to ensure
this).

Change-Id: I5fc0a5e3ffc3f4cd597d92ee685ab19568ea18f7
Signed-off-by: Pavel Chupin <pavel.v.chupin@intel.com>
diff --git a/libc/arch-x86_64/syscalls/arch_prctl.S b/libc/arch-x86_64/syscalls/arch_prctl.S
new file mode 100644
index 0000000..eaf9494
--- /dev/null
+++ b/libc/arch-x86_64/syscalls/arch_prctl.S
@@ -0,0 +1,17 @@
+/* autogenerated by gensyscalls.py */
+#include <asm/unistd.h>
+#include <linux/err.h>
+#include <machine/asm.h>
+
+ENTRY(arch_prctl)
+    movl    $__NR_arch_prctl, %eax
+    syscall
+    cmpq    $-MAX_ERRNO, %rax
+    jb      1f
+    negl    %eax
+    movl    %eax, %edi
+    call    __set_errno
+    orq     $-1, %rax
+1:
+    ret
+END(arch_prctl)