Make gensyscalls.py use the ALIAS_SYMBOL macro.

Change-Id: Ib94c0abb6fc85126ecc5ed3f1962b2b8b90b9952
diff --git a/libc/arch-x86_64/syscalls/mmap.S b/libc/arch-x86_64/syscalls/mmap.S
index 8aa4780..0c25473 100644
--- a/libc/arch-x86_64/syscalls/mmap.S
+++ b/libc/arch-x86_64/syscalls/mmap.S
@@ -15,5 +15,4 @@
     ret
 END(mmap)
 
-    .globl mmap64
-    .equ mmap64, mmap
+ALIAS_SYMBOL(mmap64, mmap)