pythonGH-131033: Enable the optimizing macros UNLIKELY and LIKELY for Clang (pythonGH-131019)

This includes clang-cl on Windows, which does not define the GCC version that was previously being checked.

Author: chris-eibl

Objects/obmalloc.c

@@ -1523,9 +1523,9 @@ PyObject_Free(void *ptr)
 }
 
 
-/* If we're using GCC, use __builtin_expect() to reduce overhead of
+/* Use __builtin_expect() where available to reduce overhead of
    the valgrind checks */
-#if defined(__GNUC__) && (__GNUC__ > 2) && defined(__OPTIMIZE__)
+#if (defined(__clang__) || (defined(__GNUC__) && (__GNUC__ > 2))) && defined(__OPTIMIZE__)
 #  define UNLIKELY(value) __builtin_expect((value), 0)
 #  define LIKELY(value) __builtin_expect((value), 1)
 #else