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

Replaced the expression computing the final result in loghelper with an
fma() call:

    result = fma(func(2.0), (double)e, func(x));

This change reduces rounding error and improves the ULP distribution of
math.log10() for very large integer inputs.

Author: Abhi210

Misc/ACKS

@@ -2161,5 +2161,6 @@ Jelle Zijlstra
 Gennadiy Zlobin
 Doug Zongker
 Peter Åstrand
+Abhishek Tiwari
 
 (Entries should be added in rough alphabetical order by last names)

Misc/NEWS.d/next/Core_and_Builtins/2025-10-22-23-26-37.gh-issue-140443.wT5i1A.rst

@@ -0,0 +1,4 @@
+Use a fused multiply–add (fma) in math.log helper to compute the
+contribution from the exponent when handling very large integer inputs. This
+reduces rounding error (fewer intermediate roundings) and produces a tighter
+ULP error distribution for functions like math.log10() and math.log()

Modules/mathmodule.c

@@ -2309,7 +2309,7 @@ loghelper(PyObject* arg, double (*func)(double))
             assert(e >= 0);
             assert(!PyErr_Occurred());
             /* Value is ~= x * 2**e, so the log ~= log(x) + log(2) * e. */
-            result = func(x) + func(2.0) * e;
+            result = fma(func(2.0), (double)e, func(x));
         }
         else
             /* Successfully converted x to a double. */