FindCygwin: use find_program instead of find_path to find 64 bit cygwin for 32 bit build and vice versa
This fix enables the use of 64 bit cygwin installation for 32 bit builds and vice versa.
The second commit contains a fix for registry value expansion with alternative WOW64 view. (revoked)
For details read the commit messages.
Topic-rename: FindCygwin-alt-arch