Fix _exit for Linux

[Roland McGrath]

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> Roland McGrath wrote:
> > I think you can just add the alias to sysdeps/unix/_exit.S instead.
> 
> No.  MIPS is one of the platforms without a functioning INLINE_SYSCALL
> implementation.  They need __syscall_* entry points.

Yes, they need a __syscall_exit alias for _exit.  Why not use an alias of
the existing _exit instead of having syscalls.list generate a different one?