Fix a bug where a floating point number was cast to int and the value was stored incorrectly because of low byte width. (#7703)

Reported in https://github.com/google/flatbuffers/issues/7690

dart/test/flex_builder_test.dart

@@ -40,6 +40,10 @@ void main() {
       flx.addInt(-1025);
       expect(flx.finish(), [255, 251, 5, 2]);
     }
+    {
+      var builder = Builder()..addDouble(1.0);
+      expect(builder.finish(), [0, 0, 128, 63, 14, 4]);
+    }
     {
       var flx = Builder();
       flx.addDouble(0.1);