add support for linux-arm64

[toolboc]

• Addresses Add support for aarch64 release builds #1911
• Adds support for linux-arm64

Note:
Current .NET 2.1.403 release @ https://dotnetcli.azureedge.net/dotnet/Sdk/2.1.403/dotnet-sdk-2.1.403-linux-arm64.tar.gz reports a runtime-id of "ubuntu.18.04-arm64" instead of an expected "linux-arm64". This needs to be updated in the published dotnet core arm64 releases before this PR will work as intended. An issue has been filed @ dotnet/sdk#2644

[wfurt]

Did you ever thought about cross-targeting @TingluoHuang ?
There is no need to run on ARM to produce ARM executables as well one can publish for all OSes without having to run on particular one. I think that would simplify release process and testing.

There also seems to be lot of duplication across multiple project files. Would it make sense to have single place to determine targetOS and targetArch and simply include it where needed?

[damccorm]

Hey @toolboc - I'm going to close here since we did this in #2881. If you think that's wrong we can reopen here.