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