The hypothetical is entirely ridiculous; just because their processor is RISC-V and their operating system is Linux won't do a thing for their missile's ability to bypass defense systems. That would be in the design of the missile and any evasion systems built into it, and I have not seen any open source hide-your-missile-from-radar codebases out there.
Let's say that China needs a massively fast chip for something. They could build their own from an open source starting point, but equally they could just copy a design from ARM, AMD, or anyone else they want. They don't do that now because they lack the capacity to manufacture it and because they can't sell it for a profit, which is why they're focusing on manufacturing more than design. If they wanted it for their own use, removing the profit motive, they could do a lot of things whether the design was freely licensed or not, ignoring all legalities with impunity.