Address PR feedback

Address PR feedback

PR feedback:
- Remove AppX checks and split Environment.Windows into Environment.Windows, Environment.CoreCLR, and Environment.NETNative
- Change marshaling of service pack version field in OSVERSIONINFOEX
- Forward TickCount to EnvironmentAugments rather than implementing it via a P/Invoke
- Move environment variables support from coreclr to corefx

Plus:
- Use GetLogicalProcessorInformationEx to get the processor count, and cache it
- Use GetUserNameExW instead of GetUserNameW, as it's available on more platforms
- Fix compilation for .NET Native
- Fixed a test

Commit migrated from https://github.com/dotnet/corefx/commit/282ad11461c480a282a529c38d8fc353bc03663b