Related #12132
Originally reported #42250 and dotnet/sdk#15225

Public signing is supported by our other compilers and should be added to ILAsm.
It should be pretty cheap. https://github.com/dotnet/corefx/blob/master/Documentation/project-docs/public-signing.md
Here's what it looks like: https://github.com/dotnet/buildtools/blob/78260239e4db68ad5c2df774a27d5f9a54fd92e5/src/Microsoft.DotNet.Build.Tasks/OpenSourceSign.cs#L71

cc @briansull