Add link to shared portion of CoreLib to .gitignore (#1607)

Add link to shared portion of CoreLib to .gitignore (#1607)

In the coreclr repo, one could simply findstr in the System.Private.CoreLib directory to find stuff.

Since in the runtime repo the shared portion of the CoreLib moved to src/libraries/, but private portion lives in src/coreclr, the files are split. We can work around it locally by making a symbolic link.

That way runtime devs that just grep/findstr stuff don't have to suffer entering the src/libraries directory where tab completion doesn't work because there's 180 directories prefixed by `System.`.