Tests, examples fail to build with GGML_BACKEND_DL=ON

When building with `GGML_BACKEND_DL=ON`, linker failures start to appear. For example, from [test-opt.c:843](https://github.com/ggml-org/ggml/blob/master/tests/test-opt.cpp#L843):

```shell
$ cmake -B build -DGGML_NATIVE=OFF -DGGML_BACKEND_DL=ON
...

$ cmake --build build
...
[ 25%] Building CXX object tests/CMakeFiles/test-opt.dir/test-opt.cpp.o
[ 26%] Linking CXX executable ../bin/test-opt
/usr/bin/ld: CMakeFiles/test-opt.dir/test-opt.cpp.o: in function `main':
test-opt.cpp:(.text.startup+0xbf): undefined reference to `ggml_backend_is_cpu'
/usr/bin/ld: test-opt.cpp:(.text.startup+0xd4): undefined reference to `ggml_backend_cpu_set_n_threads'
collect2: error: ld returned 1 exit status
gmake[2]: *** [tests/CMakeFiles/test-opt.dir/build.make:102: bin/test-opt] Error 1
gmake[1]: *** [CMakeFiles/Makefile2:633: tests/CMakeFiles/test-opt.dir/all] Error 2
gmake: *** [Makefile:146: all] Error 2
```

I was looking into a PR to fix this, but I'm not sure to best go about it, or if this even needs fixing because it's working as intended.

Using the above example, when using `GGML_BACKEND_DL`, how would upstream guide to use  `ggml_backend_cpu_set_n_threads` correctly? Should these be `dlsym`ed by the caller?

My naive approach would have been to expect that there is some shim that takes care of this and that errors out when certain functionality is not available, which could be guarded by a `ggml_backend_is_<foo>`, but the latter two seems to be defined in the modules. But I'm not that familiar with the codebase yet, so perhaps this already exists.

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