Snowflake broken if no libatomic on host

OS: Debian stretch

TB alpha 7.5a8

Tor Browser logs:

https://share.riseup.net/#H0k8k6BxXvsOGMq2CODCfg

steps to reproduce error:

1. click on configure

2. select snowflake

3. try to connect

4. error shows up (screenshot: !https://trac.torproject.org/projects/tor/attachment/ticket/24465/error-screenshot-snowflake-missing.png)