Cygwin: Use a separate Start Menu folder for WoW64 installs

This aligns the shortcuts to documentation with the setup changes in
https://sourceware.org/pipermail/cygwin-apps/2020-March/039873.html

v2:
Create/remove the Start Menu directory as needed/possible
Correctly use that directory when making shortcuts

winsup/doc/etc.preremove.cygwin-doc.sh

@@ -26,7 +26,8 @@ do
 done
 
 # Cygwin Start Menu directory
-smpc_dir="$($cygp $CYGWINFORALL -P -U --)/Cygwin"
+case $(uname -s) in *-WOW*) wow64=" (32-bit)" ;; esac
+smpc_dir="$($cygp $CYGWINFORALL -P -U --)/Cygwin${wow64}"
 
 # check Cygwin Start Menu directory still exists
 [ -d "$smpc_dir/" ] || exit 0
@@ -52,3 +53,5 @@ $site/index.html	Home\ Page	Cygwin\ Home\ Page\ Link
 $site/faq.html		FAQ	Cygwin\ Frequently\ Asked\ Questions\ Link
 EOF
 
+# remove Cygwin Start Menu directory if empty
+/usr/bin/rmdir --ignore-fail-on-non-empty "${smpc_dir}"