hw/riscv/Kconfig: Restrict NUMA to Virt & Spike machines

hw/riscv/Kconfig: Restrict NUMA to Virt & Spike machines

Only the Virt and Spike machines use NUMA. Add a RISCV_NUMA Kconfig
symbol and only have these machines select it. Adapt the Meson file
to only built it if required.

Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210515173716.358295-8-philmd@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>