Don't abort if the runtime is run twice.

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
auto merge of #9742 : alexcrichton/rust/issue-9739, r=brson

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