[OpenMP][Libomptarget] Introduce changes to support remote plugin

In order to support remote execution, we need to be able to send the
target binary description to the remote host for registration (and
consequent deregistration). To support this, I added these two
optional new functions to the plugin API:
- `__tgt_rtl_register_lib`
- `__tgt_rtl_unregister_lib`

These functions will be called to properly manage the instance of
libomptarget running on the remote host.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D93293