FlagTables: Expose /arch:NotSet to force EnableEnhancedInstructionSet to NotSet

Adds the ability to specify /arch:NotSet to force EnableEnhancedInstructionSet to NotSet for each v14#_CL.json

Further discussion here

Snippet:

I have a project that wishes to set EnableEnhancedInstructionSet to a “NotSet” value for visual studio generation, I currently have to handle this via VS_USER_PROPS, which seems a bit like a hack. On testing noticed that exposing /arch:NotSet on the relevant flag tables Templates/MSBuild/FlagTables/v143_CL.json I was able to get the desired result which seemed a lot cleaner. EnableEnhancedInstructionSet value explicitly set to NotSet in all relevant vcxproj files.