Method signbit

cpp/src/parquet/statistics_test.cc:1533–1535

template <typename T>
bool TestFloatStatistics<T>::signbit(c_type val) {
  return std::signbit(val);
}
template <>
bool TestFloatStatistics<Float16LogicalType>::signbit(c_type val) {
  return Float16::FromLittleEndian(val.ptr).signbit();