Regression since 10.8.3: accidentally broken Atomic check for macOS

Not sure what relation that got to CI times, but the change introduced in https://github.com/facebook/rocksdb/commit/1d18c4ed0177f184f228a7cdfb78eb85d0dab540 with regard to atomics is wrong and breaking.
It should be reverted to `if (NOT MSVC)`.