cmake: don't try to link with atomic on Apple
cmake: don't try to link with atomic on Apple

-latomic is necessary with Clang on Linux, but
apparently, on OSX it's not found.

eiabea/bitmonero,ranok/bitmonero,ranok/bitmonero,eiabea/bitmonero,eiabea/bitmonero,eiabea/bitmonero,eiabea/bitmonero,eiabea/bitmonero,ranok/bitmonero,ranok/bitmonero,ranok/bitmonero,ranok/bitmonero