Use NTFS junctions to share/julia/base etc instead of Cygwin symlinks

I would've committed this straight to master if @StefanKarpinski hadn't just reverted a feature addition.

This switches from using Cygwin symlinks, which only Cygwin applications can read, to NTFS junctions (as we're using in the MSYS build) for `share/julia/base`, `share/julia/doc`, and `share/julia/examples`, and `share/julia/test`. This allows the from-source Julia build in a Cygwin-to-MinGW cross-compile to reference files with the same paths relative to `JULIA_HOME` as a binary install.