Arcade: Add AsmDiff.exe to buildtools

[ericstj]

This is a commandline version of ApiReviewer and helpful for generating assembly metadata diffs.

[ericstj]

The tool is available internally at \\fxcore\toolsbin\AsmDiff.exe. Source code is at https://devdiv.visualstudio.com/DevDiv/_git/CoreFxTools?path=%2Fsrc%2Fapps%2FAsmDiff&version=GBmaster&_a=contents

[markwilkie]

What do you think should happen here @ericstj

[ericstj]

Someone can pick it up and open source it. Consider this "nice to have" on the backlog. With it we can do things like generate API diffs and dead-code diffs as part of the build.

[Anipik]

This tool has been moved to arcade, we also added the support for generating markdownfiles

[eerhardt]

https://github.com/dotnet/arcade/tree/master/src/Microsoft.DotNet.AsmDiff in case someone is looking for the link.

[ericstj]

Would be nice to plumb this into the ILLink targets now that we have a package to use.