Make gensyscalls.py use the ALIAS_SYMBOL macro.

Change-Id: Ib94c0abb6fc85126ecc5ed3f1962b2b8b90b9952
diff --git a/libc/arch-x86_64/syscalls/_exit.S b/libc/arch-x86_64/syscalls/_exit.S
index c79091d..1ab4d4f 100644
--- a/libc/arch-x86_64/syscalls/_exit.S
+++ b/libc/arch-x86_64/syscalls/_exit.S
@@ -14,5 +14,4 @@
     ret
 END(_exit)
 
-    .globl _Exit
-    .equ _Exit, _exit
+ALIAS_SYMBOL(_Exit, _exit)