As opposed to C/C++ compilers being built in C/C++?

EDIT: Try to build NetBSD for some esoteric platform from the comfort of your desktop machine. Remember to pick up your jaw from the floow when you're done. ;-) My point is, it does not have to be hard.

In my understanding, Go re-implements the system's interface, rather than using libc (or the equivalent), even on platforms where the syscall interface isn't considered stable. This means cross-compiling is really easy, but it can also mean breakage when the unstable interface changes. A tradeoff like anything else.

Rust compiler is LLVM afaik so C++, same for the JVM / C#. Having your entire runtime / tools in the same language make it easier.

This is true for Rust, however, given that LLVM is the project doing the codegen, it's a little bit special; cross-compiling it to the platforms it supports is a bit easier than a random C++ library, since it's the one with the support in the first place.

Rust projects make much more use of C libraries than Go projects do, though, so for things other than the Rust compiler, this is still a good point.

Go uses a common assembly with ways to covert into the specific architecture which makes porting the toolchain to new targets somewhat trivial. (https://www.youtube.com/watch?v=KINIAgRpkDA)