Handle the x86/x86_64 split when downloading setup.ini

Cygwin recently introduced an x86_64 version and so all the mirrors
now have x86 or x86_64 in their paths, depending on which version
you've installed.

The only change required is when the setup file is fetched.

The setup file itself contains the paths prefixed with either
x86 or x86_64
fetch setup.ini using $arch in mirror path as well

boothj5 pointed out that the script also needs the $arch
in the URL when fetching setup.ini as a fall back to
fetching setup.bz2