Is it possible to license Go source code using the AGPL? Every Go program gets linked statically with the Go runtime, which does not use the AGPL. Is this allowed to use the AGPL for my own code and link it with the Go runtime?
The Go Runtime appears to use the so-called BSD 3-Clause "New" or "Revised" License - see https://github.com/golang/go/blob/master/LICENSE
This licence is compatible with the AGPL, so releasing a work that combines novel code licensed by you under AGPL with third party code (Go runtime) licensed by others under BSD 3-Clause should be no problem. As you or others distribute the work you need to observe the terms and conditions of both licenses.
In fact as a non-copyleft license the MIT licence is compatible with almost any other licence. If you want to licence your code with a condition that all distributors must pay you £50 per copy issued, or only distribute copies on Mondays, or only distribute copies while wearing silly hats, or that no-one may distribute your code at all without express permission, you are also free to do that.