Add arm64 runtime ids for Fedora and RHEL 8

[omajid]

Both Fedora and RHEL 8 have arm64 (aka aarch64) among the primary/main
architectures they support.

Fedora includes aarch64 as a "Primary Architecture":
https://fedoraproject.org/wiki/Architectures#Primary_Architectures

RHEL 8 lists aarch64 right next to x86_64 in the developer download site:
https://developers.redhat.com/products/rhel/download

[bartonjs]

I'm not certain what the support status of .NET Core 3.0 + RHEL.8-arm64 is (or Fedora.xx-arm64); but we already have other RIDs in this graph that aren't supported... so doesn't seem like there's harm in adding it.

[bartonjs]

On the one hand, it looks like Fedora 23 didn't support arm64/aarch64 (lack of an F23 in https://fedoraproject.org/wiki/Category:AArch64). On the other hand, it probably doesn't matter, and isn't worth splitting the definition in the props file over this.

[omajid]

If you know a clean way to split versions, I would be happy to make this conditional on, say, Fedora 30 and later.

[omajid]

Not sure about support. I tried out 3.0 Preview 6 on RHEL 8:

$ ./dotnet --info
.NET Core SDK (reflecting any global.json):
 Version:   3.0.100-preview6-012264
 Commit:    be3f0c1a03

Runtime Environment:
 OS Name:     rhel
 OS Version:  8
 OS Platform: Linux
 RID:         linux-arm64
 Base Path:   /home/omajid/microsoft-build/sdk/3.0.100-preview6-012264/

Host (useful for support):
  Version: 3.0.0-preview6-27804-01
  Commit:  fdf81c6faf

.NET Core SDKs installed:
  3.0.100-preview6-012264 [/home/omajid/microsoft-build/sdk]

.NET Core runtimes installed:
  Microsoft.AspNetCore.App 3.0.0-preview6.19307.2 [/home/omajid/microsoft-build/shared/Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 3.0.0-preview6-27804-01 [/home/omajid/microsoft-build/shared/Microsoft.NETCore.App]

To install additional .NET Core runtimes or SDKs:
  https://aka.ms/dotnet-download

At least Hello World works:

 $ ../microsoft-build/dotnet new console                                               

Welcome to .NET Core 3.0!
---------------------
SDK Version: 3.0.100-preview6-012264

Telemetry
---------
The .NET Core tools collect usage data in order to help us improve your experience. The data is anonymous. It is collected by Microsoft and shared with the community. You can opt-out of telemetry by setting the DOTNET_CLI_TELEMETRY_OPTOUT environment variable to '1' or 'true' using your favorite shell.                                                  

Read more about .NET Core CLI Tools telemetry: https://aka.ms/dotnet-cli-telemetry                                   

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--------------------------------------------------------------------------------------                               
Getting ready...
The template "Console Application" was created successfully.                                                         

Processing post-creation actions...
Running 'dotnet restore' on /home/omajid/test/test.csproj...                                                         
  Restore completed in 488.57 ms for /home/omajid/test/test.csproj.                                                  

Restore succeeded.

$ ../microsoft-build/dotnet run                                                       
Hello World!

My normal builds on RHEL 8 and Fedora rely on source-build but source-build doesn't support arm64 at all, at the moment.