gh-140443: Use `fma` in `loghelper` to improve accuracy of log for very large integers (#140469)

* gh-140443:use fma in loghelper to improve accuracy of log for very large integers

Use fused multiply-add in log_helper() for huge ints.

Saving a rounding here is remarkably effective. Across some millions
of randomized test cases with ints up to a billion bits, on Windows
and using log10, the ULP error distribution was dramatically
flattened, and its range was nearly cut in half. In fact, the largest
error Tim saw was under 0.6 ULP.

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Co-authored-by: abhi210 <27881020+Abhi210@users.noreply.github.com>
Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>