[C++] Use software __builtin_popcountll when building without SSE4.2

This is to be extra safe in the context of ARROW-5381

**Reporter**: [Wes McKinney](https://issues.apache.org/jira/browse/ARROW-6663) / @wesm
#### Related issues:
- [[C++] Crash at arrow::internal::CountSetBits](https://github.com/apache/arrow/issues/21840) (is a child of)
- [[C++] signal SIGILL raised by Illegal instruction](https://github.com/apache/arrow/issues/37707)

_{**Note**: *This issue was originally created as [ARROW-6663](https://issues.apache.org/jira/browse/ARROW-6663). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}