Support for PowerPC and Z series CPU architectures

[kryptonite303]

Describe the enhancement
Support PowerPC and Z series CPU architectures for the runner

Additional information
According to the Linux System Prerequisites doc: https://github.com/actions/runner/blob/master/docs/start/envlinux.md
And this Pull Request: #184
the runner supports x64 and ARM CPU architectures.
I have jobs that run on several different architectures, including PowerPC and Z series.

[juliobbv]

For reference for future readers of this issue: the actions runner uses .NET Core as a dependency. That means, as of right now, we can only support the OS/CPU architectures that .NET Core itself currently supports.

https://github.com/dotnet/core/blob/master/release-notes/3.1/3.1-supported-os.md

Porting the runner to Go or Typescript will allow us to support a wider variety of OS/CPU arch combos.

[bryanmacfarlane]

Dupe of #243 - please add a comment to that issue if you have specific requests.