Calling libc::exit immediately after startup causes assertion failure in rust_initialize_rt_tls_key

[brson]

As in #8870. To fix that we stopped calling exit, but there's a deeper problem that might be fixable.

[huonw]

The following runs fine on Linux, but, I don't recall ever being able to reproduce #8870 on this computer.

fn main() {
    println!("hi");
    unsafe { std::libc::exit(0) }
}

It seems unlikely that we ever accidentally fixed the fundamental problem, but rust_initialize_rt_tls_key disappeared in e8bf078, so I'm closing this.