Fix install and win-extras for Cygwin cross

- call cygpath_w on julia executable for stringpatch
- don't try to copy p7zip bin and lib into dist-extras on Cygwin
- chmod a+x on both 7z.exe and 7z.dll, before first running 7z.exe
Merge pull request #6045 from tkelman/patch-3

Fix install and win-extras for Cygwin cross