OpenBSD support.

[3405691582]

On this platform, the TIOCGWINSZ ioctl identifier is a complex macro.
Since we don't have a C bridging header obviously available to get the
flattened value, supply is the flattened value obtained elsewhere. This
is of course brittle but this is the simplest way around this for now.

Additionally, ensure we support the platform architecture name, where
the x86_64 architecture is called amd64 instead.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
  • not trivially testable
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary
  • not covered

[natecook1000]

Thanks for this, @3405691582!

[natecook1000]

@swift-ci Please test

[3405691582]

Linux test failed for timeout. Maybe transient, could you restart?

[kylemacomber]

@swift-ci please test