Not sure if that's just a typo, but you would use a signed char, which is a different type to char even on implementations where char is signed. Part of the reason for this, of course, is because char can be unsigned so if you want a signed integer you have to specify that. But more philosophically, unsigned char and signed char are numerical types that are not meant to be characters (despite their names), whereas char is a character type that just happens to be backed by an integer.

Indeed I believe that int8_t is almost always just a typedef for signed char (but I still would use int8_t where available for clarity).