Merge pull request #6489 from JohnChen0/master

Update CrossGen doc for "Unable to load Jit" error

Author: John Chen

Documentation/building/crossgen.md

@@ -63,3 +63,4 @@ The following are some of the command errors while creating or using native imag
 - "Could not load file or assembly 'mscorlib.dll' or one of its dependencies. The native image could not be loaded, because it was generated for use by a different version of the runtime. (Exception from HRESULT: 0x80131059)": This error indicates that there is a mismatch between CrossGen and mscorlib.ni.dll. Make sure to use CrossGen and mscorlib.ni.dll from the same build or NuGet package.
 - "Error: Could not load file or assembly '...' or one of its dependencies. The system cannot find the file specified. (Exception from HRESULT: 0x80070002)": CrossGen wasn't able to find a particular dependency that it needs. Verify that you have the assembly specified in the error message, and make sure its location is included in `/Platform_Assemblies_Paths`.
 - CoreCLR unable to initialize: While there are many possible causes of this error, one possibility is a mismatch between mscorlib.ni.dll and coreclr.dll (or libcoreclr.so). Make sure they come from the same build or NuGet package.
+- "Unable to load Jit Compiler": Please get a copy of `clrjit.dll` (or `libclrjit.so` or `libclrjit.dylib`, depending on your platform), and place it in the same directory as CrossGen. You can either build `clrjit.dll` yourself, or get it from `Microsoft.NETCore.Jit` NuGet package. To avoid possible issues, please use `clrjit.dll` from the same build as `crossgen.exe` if possible.