Link against libatomic if needed

[roehling]

Some architectures such as RISC-V have only limited support for atomic
operations and need the GCC atomic helper library for
compare_exchange_strong() to work.

This PR adds a config-time test to check if libatomic is required and
links against it if yes.

[codecov]

Codecov Report

Merging #95 (08b9947) into master (ac7412a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #95   +/-   ##
=======================================
  Coverage   70.14%   70.14%           
=======================================
  Files          83       83           
  Lines        4217     4217           
=======================================
  Hits         2958     2958           
  Misses       1259     1259           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ac7412a...08b9947. Read the comment docs.

[foonathan]

Thanks.