Fix install and win-extras for Cygwin cross

- call cygpath_w on julia executable for stringpatch (MinGW compiled stringpatch exe doesn't understand cygwin paths)
- don't try to copy p7zip bin and lib into dist-extras on Cygwin (@vtjnash, can you recall why this step is necessary in a Linux-to-MinGW cross compile?)
- chmod a+x on both 7z.exe and 7z.dll, before first running 7z.exe