Optimizing a faster isdigit() about llvm-mos-sdk

[TheHans255]

I would suggest replacing ':`` with '9' + 1inisdigit_2`, like so:

bool is_digit2(unsigned char ch)
{
  ch -= ('9' + 1);
  return ch >= 245;
}

and adapting isdigit_3 accordingly. While it is a good assumption in most character sets to assume that '0' through '9' are contiguous and in order, ':' coming immediately after '9' is specific to ASCII, which 65XX platforms (especially game consoles) don't always use.