[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.}

[rvschaffer]

Hi @asfimport , I'm on old hardware enthusiast (haha), and this issue is really a showstopper for me!
There are many other in the same situation.
Please make Arrow usable on "popcnt'less" CPUs, so we can continue using them.

[abcbarryn]

I see this still hasn't been fixed. Is Pyarrow no longer maintained?