DRAFT test: 16-cast.c: Fix for riscv64.

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

@@ -21,7 +21,7 @@
 int
 main ()
 {
-  char c = -1;
+  signed char c = -1;
   unsigned u = c;
   unsigned t = (unsigned char)c;
   int i = -1;