Restore missing native *.ni.pdb file from the Microsoft.NETCore.Runtime.CoreCLR ...

Restore missing native *.ni.pdb file from the Microsoft.NETCore.Runtime.CoreCLR (#12677)

* Restore missing native *.ni.pdb file from the Microsoft.NETCore.Runtime.CoreCLR package

DLLs that have native code, need both the *.pdb (for IL information) and the *.ni.pdb (for
native information).

When System.Private.Corlib was renamed from System.Private.Corlib.ni.pdb to System.Private.Corlib,
the logic that decides what goes into the  Microsoft.NETCore.Runtime.CoreCLR package lost
the *.ni.pdb for System.Private.Corlib.

This change basically causes this logic to search both for *.ni.pdb as well as the *.pdb file and
thus fixes this issue.

* Remove unnecessary condition (review feedback)