SignTool needs to avoid attempting to local strong name sign if the binary is already strong name signed

[mmitche]

On Mac/Linux, we can't use sn.exe. This should be fine, as local strong name signing should be done during compilation. However, when a file is not authenticode signed, signtool wants to strip away the sn-sign bit and then re-sign using sn.exe.

We need to avoid this if the file is already signed during compilation.

[mmitche]

Completed with #15309