Consider adding `[NonVersionable]` to OS and process architecture getters.

[teo-tsirpanis]

Context: #129926 (comment)

In the context of an R2R compilation, the operating system and process architecture are constant values, aren't they? If so, it might be a good idea to mark [NonVersionable] the APIs to get the operating system or process architecture, in order to allow more dead code elimination in R2R. Specifically, I'm asking for the following APIs:

• OperatingSystem.Is***()
  • I am not including the Is***VersionAtLeast APIs, because the OS version is not known at publish time. As a separate work item, crossgen could expand them as Is***() && Is***VersionAtLeast(x, y, z) to eliminate dead code if we are compiling for a completely different OS.
• RuntimeInformation.ProcessArchitecture

We could also mark these APIs, but they are superseded by OperatingSystem.Is*** so it might not be needed:

• The OSPlatform type.
• RuntimeInformation.IsOSPlatform

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[jkotas]

RuntimeInformation.ProcessArchitecture

Sounds reasonable.

OperatingSystem.Is***()

Is*** methods for concrete platforms like IsMacOS are marked as NonVersionable already. I assume that you mean bool IsOSPlatform(string platform). I doubt that marking that with NonVersionable is going to enable dead-code elimination without additional changes in crossgen2 or JIT - StringComparison.OrdinalIgnoreCase is a pain to deal with.

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[LE0-Lin]

I'd like to work on the accepted RuntimeInformation.ProcessArchitecture scope. I'll add the NonVersionable annotation and verify its effect on ReadyToRun architecture-specific dead-code elimination.