gnu atomics has been removed since version 3.5.27:
https://github.com/squid-cache/squid/commit/ddd4edb743d82be97fc651d529e04bf55329a50d
So remove squid_cv_gnu_atomics handling
Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com> Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>