Use of "GNU/Linux" and "Linux"

[JayFoxRox]

In https://xqemu.com/getting-started/#building-on-gnulinux we use "GNU/Linux" (other instances might exist too). While "GNU/Linux" is probably the correct thing to do, it also doesn't match the "Linux" form we use pretty much anywhere else in the XQEMU ecosystem.

I prefer "Linux" for the simplicity.
I'm fine with using "GNU/Linux" if somebody is absolutely desperate about it, but otherwise I'd say we should stick to just using "Linux" and all existing instances of "GNU/Linux" should be changed.