Fix `FIXABLE_FLOAT()` on `MRB_INT64`; ref #4566

Normal `TYPED_FIXABLE(f,mrb_float)` does not work on 64bit int from
casting problems. The new approach works well, but assumes two's
complement and IEEE-754 floating point numbers.

include/mruby/numeric.h

@@ -23,8 +23,12 @@ MRB_BEGIN_DECL
 #define NEGFIXABLE(f) TYPED_NEGFIXABLE(f,mrb_int)
 #define FIXABLE(f) TYPED_FIXABLE(f,mrb_int)
 #ifndef MRB_WITHOUT_FLOAT
+#ifdef MRB_INT64
+#define FIXABLE_FLOAT(f) ((f)>=-9223372036854775808.0 && (f)<9223372036854775808.0)
+#else
 #define FIXABLE_FLOAT(f) TYPED_FIXABLE(f,mrb_float)
 #endif
+#endif
 
 #ifndef MRB_WITHOUT_FLOAT
 MRB_API mrb_value mrb_flo_to_fixnum(mrb_state *mrb, mrb_value val);