Implement _Exit(3).

Change-Id: Ida6ac844cc87d38c9645b197dd8188bb73e27dbe
diff --git a/libc/arch-arm/syscalls/_exit.S b/libc/arch-arm/syscalls/_exit.S
index 1a6b35d..581b340 100644
--- a/libc/arch-arm/syscalls/_exit.S
+++ b/libc/arch-arm/syscalls/_exit.S
@@ -12,3 +12,6 @@
     neg     r0, r0
     b       __set_errno
 END(_exit)
+
+    .globl _Exit
+    .equ _Exit, _exit