pythongh-111139: Optimize math.gcd(int, int)

Add a fast-path for the common case.

Benchmark:

    python -m pyperf timeit \
        -s 'import math; gcd=math.gcd; x=2*3; y=3*5' \
        'gcd(x,y)'

Result: 1.07x faster (-3.4 ns)

    Mean +- std dev: 52.6 ns +- 4.0 ns -> 49.2 ns +- 0.4 ns: 1.07x faster

Modules/mathmodule.c

@@ -765,6 +765,16 @@ math_gcd(PyObject *module, PyObject * const *args, Py_ssize_t nargs)
     if (nargs == 0) {
         return PyLong_FromLong(0);
     }
+
+    // Fast-path for the common case: avoid calling _PyNumber_Index()
+    // and the loop.
+    if (nargs == 2
+        && PyLong_CheckExact(args[0])
+        && PyLong_CheckExact(args[1]))
+    {
+        return _PyLong_GCD(args[0], args[1]);
+    }
+
     res = PyNumber_Index(args[0]);
     if (res == NULL) {
         return NULL;