PPC: return failure from the unw_get_proc_info() stub

Stop pretending that unw_get_proc_info() works on PPC, and instead give
an error back to the caller. As far as I can tell, none of the libunwind
tests clear out the `unw_proc_info_t' before passing it to
unw_get_proc_info(), so they would end up working on garbage data.

src/ppc/Gget_proc_info.c

@@ -30,5 +30,5 @@ PROTECTED int
 unw_get_proc_info (unw_cursor_t *cursor, unw_proc_info_t *pi)
 {
   /* XXX: empty stub.  */
-  return 0;
+  return -UNW_EINVAL;
 }