Add missing aliases for off64_t functions in 64-bit land.

See the comment in SYSCALLS.TXT for an explanation.

Change-Id: I33d4056e84160c3cca74b7b588e9924a569753ed
diff --git a/libc/arch-x86_64/syscalls/sendfile.S b/libc/arch-x86_64/syscalls/sendfile.S
index 565276e..4aa2a31 100644
--- a/libc/arch-x86_64/syscalls/sendfile.S
+++ b/libc/arch-x86_64/syscalls/sendfile.S
@@ -17,3 +17,6 @@
 1:
     ret
 END(sendfile)
+
+    .globl _C_LABEL(sendfile64)
+    .equ _C_LABEL(sendfile64), _C_LABEL(sendfile)