Don't abort if the runtime is run twice.

[alexcrichton]

This changes an assert_once_ever! assertion to just a plain old assertion
around an atomic boolean to ensure that one particular runtime doesn't attempt
to exit twice.

Closes #9739

[drrb]

Thanks for getting to this so quickly! I have a question about the fix. It seems like this was the only use of assert_once_ever in the libraries. Is this a general problem that will occur with all uses of assert_once_ever in the future? If so, would it be worth removing the macro, or modifying it to be assert_once_for_this_runtime_invocation (in the spirit of #7748)?

[brson]

Can we remove the unused macro too please?

[alexcrichton]

Updated with the macro removed, it may still have a use when doing something like initializing an FFI library, but I agree that it should probably be removed to prevent something like this in the future.