RISC-V64 has `unsigned char` for `char`, so it failed to build. In amd64 `char` is `signed char`, so the code wasn't portable. We have to be explicit about the sign of the `char`. * lib/test/scaffold/16-cast.c: Add `signed` to `char`.
* lib/tests/scaffold/16-cast.c: Add file. * build-aux/check-mescc.sh (mes_tests): Add it.