[llvm] [Offload][OpenMP] Fix RPC callback registration when RPC server is not initialized (PR #210215)

[Joseph Huber via llvm-commits]

jhuber6 wrote:

> This happens in practice if you initialize plugin before Fortran `ProgramStart`.

Any way we can make it not do that?

https://github.com/llvm/llvm-project/pull/210215