Windows drive letter is added to target prefix path when cross compiling

After fix for QTBUG-50839, using separate paths for -extprefix (host path) and -prefix (target path) no longer work correctly when cross compiling e.g., for QNX on Windows host.

The prefix path is now resolved to absolute path using Windows rules, so it ends up with the drive letter. This  is unnecessary when the extprefix is used for all the Windows host paths and it breaks binaries as they will try to use Windows path on the target.