Clang expects __INTx_TYPE__ to be used with signed/unsigned modifier
when defining (u)intx_t, GCC has a separate __UINTx_TYPE__. Newer Clang
will also provide the latter, but expects the behavior of the former to
remain. As ARM uses unsigned char by default, distinguish between Clang
and non-Clang here to the right type for int8_t.

(joerg)