Bug#948109: z3: FTBFS on riscv64, needs -latomic, blocks rustc:riscv64

[Fabian Wolff]

Thank you for your MR, Ximin!

Do you need anything else? I see you haven't added -pthread anywhere yet; is
this not necessary with CMake (maybe CMake adds it automatically?), and have
you tried building on/for riscv64 again to verify that it works now?