Add SSE4.2 to dynamic dispatch

Author: AntoinePrv

cpp/src/arrow/util/bpacking.cc

@@ -34,9 +34,11 @@ struct UnpackDynamicFunction {
 
   static constexpr auto implementations() {
     return std::array {
-      // Current SIMD unpack algorithm works terribly on SSE4.2 due to lack of variable
-      // rhsift and poor xsimd fallback.
+#if defined(ARROW_HAVE_SSE4_2)
+      Implementation{DispatchLevel::NONE, &unpack_sse4_2<Uint>},
+#else
       Implementation{DispatchLevel::NONE, &unpack_scalar<Uint>},
+#endif
 #if defined(ARROW_HAVE_RUNTIME_AVX2)
           Implementation{DispatchLevel::AVX2, &unpack_avx2<Uint>},
 #endif