Use same compiler in all corefx builds

[chcosta]

In corefx, on windows, the default is still too use the shared compiler, you have to opt into using this Roslyn compiler via "UseSharedCompilation=false". On Linux, the default is to use this Rosslyn compiler.

All corefx builds should be using the same compiler so that available features across builds are identical.

dotnet/buildtools#947

[mellinoe]

@chcosta Any update on this one?

[mellinoe]

I'm marking this up for grabs.

[wtgodbe]

I believe this has been resolved as of dotnet/corefx#24076, @chcosta please reopen if I'm mistaken