Make default IV_MUL_OVERFLOW_IS_EXPENSIVE reasonable on LLP64 platforms

This will affect Win32 x86-64.
Thanks to @tonycoz for figuring this out.

(intended to be squashed before merge)

Author: t-a-k

inline.h

@@ -3425,8 +3425,10 @@ S_lossless_NV_to_IV(const NV nv, IV *ivp)
 #  endif
 #  ifndef IV_MUL_OVERFLOW_IS_EXPENSIVE
 /* Strict overflow check for IV multiplication is generally expensive
- * when IV is a multi-word integer.  */
-#    define IV_MUL_OVERFLOW_IS_EXPENSIVE (IVSIZE > LONGSIZE)
+ * when IV is a multi-word integer.
+ * We assume that PTRSIZE matches the platform word size; LONGSIZE might not
+ * match for LLP64 platforms such as Win32 x86-64.  */
+#    define IV_MUL_OVERFLOW_IS_EXPENSIVE (IVSIZE > PTRSIZE)
 #  endif
 
 #  if defined(I_STDCKDINT) && !IV_ADD_SUB_OVERFLOW_IS_EXPENSIVE