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

[asfimport]

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

Reporter: Wes McKinney / @wesm

Related issues:

• [C++] Crash at arrow::internal::CountSetBits (is a child of)
• [C++] signal SIGILL raised by Illegal instruction

_{Note: This issue was originally created as ARROW-6663. Please see the migration documentation for further details.}