WIP: Build for all TFMs in the makefile

[baronfel]

Building off of #6181 this MR continues the quest to get this repo building significant portions of the code for all supported TFMs via the Makefile. With these changes I can run make and restore, build proto, build fsc/fsi, and build the tests on my OSX machine for all supported TFMs in those projects.

The only caveat to this is that the tests hang in the Async portion for me currently, not quite sure if that's mono/netcore or environmental.

There really wasn't a lot of work necessary after #6181, the main piece of work was adding in the missing Mono Facades directory to the search path in a way that meshed with the other properties. I had to do a bit of hackiness (which I explained with a comment) to get the search path listings to evaluate at the correct point in time.

I'm sending this up now as a sanity-check and to see how your CI works with this, as well as to solicit opinions!

[KevinRansom]

@baronfel could you hold of on doing to much of this for a few weeks. @brettfo is doing a chunk of work to integrate our build with the corclr/cli/asp build mechanism.

It's not clear to me how it is going to end up looking, or whether it's going to support mono build intrinsically rather than as an addon like it is now.

[baronfel]

Sure, can do. I'm just trying to smooth the integration paths, so it makes sense to wait a bit until that settles so I don't have to learn the whole system twice!

[baronfel]

Ok so now that the arcade work is done I've updated this makefile to plug into the expectations there (mostly around fslex/fsyacc being copied as part of the pre-build). I still build and test locally just fine for net46 and netcoreapp2.0 on mac, so that makes me happy :)

[baronfel]

At the same time I should look at what the eng/build.sh script is doing to see if that's the way to go forward.

[baronfel]

So I guess the question in my mind is that currently there's no linux/mac build that attempts to build the repo for full framework. The new dotnet arcade stuff only builds the netcore libraries and binaries, and I think we need to do more than that. Should I add builds of the makefile (which builds for both targets on mono) to the azure CI pipelines? Or is the right way to go forward to expand the arcade stuff to build for full framework?

[KevinRansom]

Thanks for taking the time to prepare this, however, I don't think we need this right now.

I'm closing this

Kevin