SERVER-12461 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.

There are obviously other ways this could be fixed; obvious ones include changing the tolerance in the test or disabling fp-contract on the command line for all files.  But this one works.