[C++] Provide non-SSE4 versions of functions that use CPU intrinsics for older processors

[asfimport]

follow up to discussion in patch for ARROW-33

Reporter: Wes McKinney / @wesm
Assignee: Antoine Pitrou / @pitrou

_{Note: This issue was originally created as ARROW-536. Please see the migration documentation for further details.}

[asfimport]

Uwe Korn / @xhochy:
The current code (as in ARROW-33) will compile and work on non-SSE4 system but the popcnt instruction will only be used by the compiler if the flag -msse4.2 is provided. We would need rather two variants of specific functions compiled, one with SSE4.2 instructions and one without. Loading the correct one must be resolved at runtime.

[asfimport]

Antoine Pitrou / @pitrou:
As part of ARROW-2653 I am making SSE4.2 mandatory on x86-64 systems. So this issue can probably be closed once the PR gets merged.

[asfimport]

Wes McKinney / @wesm:
+1