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/arch-mips/syscalls/_exit_thread.S b/libc/arch-mips/syscalls/__exit.S
similarity index 77%
rename from libc/arch-mips/syscalls/_exit_thread.S
rename to libc/arch-mips/syscalls/__exit.S
index 1ed8ffc3..529e49c 100644
--- a/libc/arch-mips/syscalls/_exit_thread.S
+++ b/libc/arch-mips/syscalls/__exit.S
@@ -2,11 +2,11 @@
 
 #include <asm/unistd.h>
     .text
-    .globl _exit_thread
+    .globl __exit
     .align 4
-    .ent _exit_thread
+    .ent __exit
 
-_exit_thread:
+__exit:
     .set noreorder
     .cpload $t9
     li $v0, __NR_exit
@@ -20,4 +20,4 @@
     j $t9
     nop
     .set reorder
-    .end _exit_thread
+    .end __exit