util: avoid non-portable __WORDSIZE

Lets not unnecessarily rely on __WORDSIZE, which is not clearly specified
by any spec. Use explicit size comparisons if we're not interested in the
WORDSIZE, anyway.

(David: adjust commit message to explain why we do this)

src/shared/util.h

@@ -205,7 +205,7 @@ int safe_atod(const char *s, double *ret_d);
 
 int safe_atou8(const char *s, uint8_t *ret);
 
-#if __WORDSIZE == 32
+#if LONG_MAX == INT_MAX
 static inline int safe_atolu(const char *s, unsigned long *ret_u) {
         assert_cc(sizeof(unsigned long) == sizeof(unsigned));
         return safe_atou(s, (unsigned*) ret_u);