Implement _Exit(3).

Change-Id: Ida6ac844cc87d38c9645b197dd8188bb73e27dbe
diff --git a/libc/arch-mips/syscalls/_exit.S b/libc/arch-mips/syscalls/_exit.S
index 5a0877d..f546b66 100644
--- a/libc/arch-mips/syscalls/_exit.S
+++ b/libc/arch-mips/syscalls/_exit.S
@@ -17,3 +17,6 @@
     nop
     .set reorder
 END(_exit)
+
+    .globl _Exit
+    .equ _Exit, _exit