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/ftruncate.S b/libc/arch-x86_64/syscalls/ftruncate.S
index 2560ade..5157fec 100644
--- a/libc/arch-x86_64/syscalls/ftruncate.S
+++ b/libc/arch-x86_64/syscalls/ftruncate.S
@@ -16,3 +16,6 @@
 1:
     ret
 END(ftruncate)
+
+    .globl _C_LABEL(ftruncate64)
+    .equ _C_LABEL(ftruncate64), _C_LABEL(ftruncate)