build: don't use -latomic on macOS

Fixes build when using open-source clang.

Builds using Apple clang from Xcode v7 and later were already working,
due to the fact that node's build system misidentifies their
llvm_version as "0.0" and thus the flag wasn't being added.

PR-URL: https://github.com/nodejs/node/pull/30099
Fixes: https://github.com/nodejs/node/issues/30093


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>