MRB_DISABLE_STDIO: String#upto native's mrb_str_upto requires snprintf

[alexsnaps]

I think this is kind of the equivalent of #3632
6fdf2b0 introduced a dependency to snprintf again that's not available with the MRB_DISABLE_STDIO defined. Maybe having a CI config that tests MRB_DISABLE_STDIO would avoid these in the future, wdyt?

[matz]

It may be too late for 1.3.

[ksss]

Oh... I'm sorry.