Improve crossbuild inference

[am11]

This should cover freebsd-on-linux etc. type of rootfs cases.

Thanks to @tmds for suggesting it dotnet/sdk#48591 (comment).

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[am11]

@ViktorHofer, could you please take a look? We decided to not added --cross as it was almost auto-inferred except for the ROOTFS case where OS are different (e.g. freebsd on linux).

[ViktorHofer]

Can we please hold this one until dotnet/dotnet#176 goes in? I expect that to happen this week. That moves the remaining DotNetBuild.props arguments into the VMR.

Asking as we don't have a property like PortableOS there and TargetOS gets set differently in the VMR. I.e. it is linux-bionix or linux-musl which it isn't the case for runtime from what I understand.

[am11]

Sure, this is actually similar to #115272 (using PortableOS to infer the cross-ness). It shouldn't interfere with TargetOS or anything else.

[ViktorHofer]

@am11 I'm closing this PR as runtime's DotNetBuild.props file got removed with dotnet/dotnet@2774010 (the change will flow into runtime tomorrow).

That arg is now here: https://github.com/dotnet/dotnet/blob/025e4e597746eafc5a736195b66aeb321455d64c/repo-projects/runtime.proj#L27

If this still needs an update, would you mind submitting a PR to dotnet/dotnet (we don't yet advertise that possibility, so please keep that scoped to just that one PR and change for now)? Thanks in advance