Support hot cold splitting in crossgen2

[cshung]

This is a PR created by merging the code from the feature branch on runtimelab.

@amanasifkhalid
@EugenioPena
@dotnet/crossgen-contrib

[trylek]

Thanks Andrew, I believe you, Aman and Eugenio have done a great job, overall this looks great to me, I haven't noticed any blocking issues, I have shared a couple of suggestions for local code cleanups and deduplications and I have a few questions regarding bits of code I have a hard time to comprehend but that's about it.

[trylek]

Please also note that there are some conflicts you'll need to resolve. For lab testing, I suppose we should probably tweak the Crossgen2 pipelines to use the hot-cold code splitting and run the crossgen2 outerloop pipeline with this change.

[trylek]

I should probably also emphasize that I mostly focused on Crossgen2 and runtime parts of the change, while I think I've been able to mostly follow the intent of the JIT changes, these definitely merit reviewing by a better JIT expert than me.

[davidwrighton]

This generally looks good. I'm looking forward to the tweaks from in response to code review.

[BruceForstall]

JIT changes LGTM