makefiles: `make term` does not print `stderr` output of child process.

If the child process, i.e., the RIOT executable on native, errors out early, e.g., due to missing dynamic libraries, nothing gets printed. Result: confused user.

Example: Remove a library from your `LD_LIBRARY_PATH`/`DYLD_LIBRARY_PATH` and observe the dylib/so loader complaint going unnoticed because standard error is not redirected by `make term`.