Revert "Lose the hand-written futex assembler."

This reverts commit ced906c849704f379d7191822f6d74993d4fa296.

Causes issues on art / dalvik due to a broken return value
check and other undiagnosed issues.

bug: 15195455

Change-Id: I5d6bbb389ecefb0e33a5237421a9d56d32a9317c
diff --git a/libc/arch-x86_64/syscalls/futex.S b/libc/arch-x86_64/syscalls/futex.S
new file mode 100644
index 0000000..62f64bd
--- /dev/null
+++ b/libc/arch-x86_64/syscalls/futex.S
@@ -0,0 +1,17 @@
+/* Generated by gensyscalls.py. Do not edit. */
+
+#include <private/bionic_asm.h>
+
+ENTRY(futex)
+    movq    %rcx, %r10
+    movl    $__NR_futex, %eax
+    syscall
+    cmpq    $-MAX_ERRNO, %rax
+    jb      1f
+    negl    %eax
+    movl    %eax, %edi
+    call    __set_errno
+    orq     $-1, %rax
+1:
+    ret
+END(futex)