[clang-offload-wrapper] Lower priority of __tgt_register_lib in favor of __tgt_register_requires

Lower priority of __tgt_register_lib in order to make sure that __tgt_register_requires is called before loading a libomptarget plugin.
We want to know beforehand which requirements the user has asked for so that upon loading the plugin libomptarget can report how many devices there are that can satisfy these requirements.

Differential Revision: https://reviews.llvm.org/D75223