Earlier quoted context omitted.
I don't follow. You can load every JITC I know of with C or C++ interfaces. And what is a JITC if not a runtime?
The runtime I'm talking about is for the language that the JIT compiler is written in. If I want to use a JIT compiler that is written in Java in my C++ game I now have to distribute the Java runtime with my game. That's an example of "not easily embeddable".
1. If by runtime you mean "set of files" you can (these days) compile the Graal JIT compiler to a fully standalone DSO/DLL or even statically link the results into a larger executable. And because the Graal JIT understands Truffle that means making standalone statically linkable JITCs for any language is now very easy.
But that said, games always consist of more than one file anyway so the importance of this doesn't seem too high.
2. If by runtime you mean "support code" then C, C++ and Rust all have runtimes so you'd really need to define what specifically you need from a runtime.