target/riscv: Only build KVM guest with same wordsize as host

target/riscv: Only build KVM guest with same wordsize as host

Per Anup Patel in [*]:

 > Currently, we only support running rv64 guest on rv64 host
 > and rv32 guest on rv32 host.
 >
 > In the future, we might support running rv32 guest on rv64
 > host but as of now we don't see a strong push for it.

Therefore, when only using the KVM accelerator it is pointless
to build qemu-system-riscv32 on a rv64 host (or qemu-system-riscv64
on a rv32 host). Restrict meson to only build the correct binary,
avoiding to waste ressources building unusable code.

[*] https://lore.kernel.org/qemu-devel/CAAhSdy2JeRHeeoEc1XKQhPO3aDz4YKeyQsPT4S8yKJcYTA+AiQ@mail.gmail.com/

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230627143235.29947-3-philmd@linaro.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>