rustc_codegen_ssa: fix get_rpath_relative_to_output panic when lib only contains file name
          #125861
        
          
      
  Add this suggestion to a batch that can be applied as a single commit.
  This suggestion is invalid because no changes were made to the code.
  Suggestions cannot be applied while the pull request is closed.
  Suggestions cannot be applied while viewing a subset of changes.
  Only one suggestion per line can be applied in a batch.
  Add this suggestion to a batch that can be applied as a single commit.
  Applying suggestions on deleted lines is not supported.
  You must change the existing code in this line in order to create a valid suggestion.
  Outdated suggestions cannot be applied.
  This suggestion has been applied or marked resolved.
  Suggestions cannot be applied from pending reviews.
  Suggestions cannot be applied on multi-line comments.
  Suggestions cannot be applied while the pull request is queued to merge.
  Suggestion cannot be applied right now. Please check back later.
  
    
  
    
When compiles program with
-C rpath=yesbut with no output filename specified, or with filename ONLY, we will get an ICE for now. Fix it by treat emptyoutputpath inget_rpath_relative_to_outputas current dir.Before this patch:
All those examples work after the patch.
Close #119571.
Close #125785.