Do not let GCC use FMA instructions for DistributionEstimators& operator <<

One of the descriptive_stats tests (TEST(SummaryEstimators,
TestNominalResults)) fails on arm64 because the error between the estimated
quantile and the true value is ever so slightly over the tolerance.  Disabling
gcc's use of fused-multiply add instructions for DistributionEstimators&
operator << brings the error back within the tolerance.