[sdks] Use @loader_path for mono and libmonosgen (#15580)
* [sdks] Use @loader_path for mono and libmonosgen
This way we are able to dynamically load libmonosgen and also use host
mono binary.
The library dependencies look like this after the change:
```
otool -L /Users/rodo/git/mono/sdks/out/android-host-Darwin-release/bin/mono
/Users/rodo/git/mono/sdks/out/android-host-Darwin-release/bin/mono:
@loader_path/libmonosgen-2.0.dylib (compatibility version 2.0.0, current version 2.0.0)
...
otool -L /Users/rodo/git/mono/sdks/out/android-host-Darwin-release/lib/libmonosgen-2.0.dylib
/Users/rodo/git/mono/sdks/out/android-host-Darwin-release/lib/libmonosgen-2.0.dylib:
@loader_path/libmonosgen-2.0.dylib (compatibility version 2.0.0, current version 2.0.0)
...
```