Fix scripts failure when KRE is installed under Program Files
Configure KRE to enable the latest behavior for back compat

 - Enable the CoreCLR host to pass the right compat switch
 - Set the .Net 4.6 TFM for the desktop host

#931
Merge branch 'LatestBehaviorNoTFM' of https://github.com/AlexGhiondea/KRuntime into LatestBehaviorNoTFM