Fix the exit syscall stub's name.

I've left the exit_group syscall as _exit because otherwise we'd have to
convince the compiler that our _exit (which just calls __exit_group) is
actually "noreturn", and it seems like that would be less clean than just
cutting out the middleman.

We'll just have to trust ourselves not to add anything to SYSCALLS.TXT
that ought to be private but that only has a single leading underscore.
Hopefully we can manage that.

Change-Id: Iac47faea9f516186e1774381846c54cafabc4354
diff --git a/libc/bionic/bionic_clone.c b/libc/bionic/bionic_clone.c
index b603a3a..8a17e13 100644
--- a/libc/bionic/bionic_clone.c
+++ b/libc/bionic/bionic_clone.c
@@ -39,14 +39,14 @@
                            int            (*fn)(void *),
                            void          *arg);
 
-extern void _exit_thread(int status);
+extern void __exit(int status);
 
 /* this function is called from the __bionic_clone
  * assembly fragment to call the thread function
  * then exit. */
 extern void __bionic_clone_entry(int (*fn)(void*), void* arg) {
   int status = (*fn)(arg);
-  _exit_thread(status);
+  __exit(status);
 }
 
 int clone(int (*fn)(void*), void* child_stack, int flags, void* arg, ...) {