Make Integer#round spec work regardless of pointer size

The spec is actually testing a behaviour stemming from NUM2INT(), and
since `sizeof(long)>=sizeof(int)`, `min_long-1` always makes NUM2INT()
raise `RangeError`.

spec/ruby/core/integer/round_spec.rb

@@ -21,10 +21,8 @@
     (-25 * 10**70).round(-71).should eql(-30 * 10**70)
   end
 
-  platform_is_not wordsize: 32 do
-    it "raises a RangeError when passed a big negative value" do
-      -> { 42.round(fixnum_min) }.should raise_error(RangeError)
-    end
+  it "raises a RangeError when passed a big negative value" do
+    -> { 42.round(min_long - 1) }.should raise_error(RangeError)
   end
 
   it "raises a RangeError when passed Float::INFINITY" do