Fixed gacutil so assemblies with the ".exe" extension can be uninstalled.

This patch fixes the Uninstall method in the `gacutil` tool so assemblies with the `.exe` extension can be uninstalled from the GAC. Without this patch, `.exe` assemblies can be installed into the GAC, but attempting to uninstall them crashes `gacutil`.

These changes are released under the MIT/X11 license.