Implement _Exit(3).

Change-Id: Ida6ac844cc87d38c9645b197dd8188bb73e27dbe
diff --git a/libc/arch-x86_64/syscalls/_exit.S b/libc/arch-x86_64/syscalls/_exit.S
index f40cfe3..3c15897 100644
--- a/libc/arch-x86_64/syscalls/_exit.S
+++ b/libc/arch-x86_64/syscalls/_exit.S
@@ -14,3 +14,6 @@
 1:
     ret
 END(_exit)
+
+    .globl _Exit
+    .equ _Exit, _exit