* Generate targets files that throw for unsupported NS tfm

For S.D.Common, S.Speech and S.Sec.Crypto.Pkcs we manually added a
targets file to mark the .NETStandard asset as not applicable.
This was done to allow defining a minimum supported .NETCoreApp
version, even though a compatible .NETStandard asset is available.
This commit automatically generates that targets file based on
items.