target/riscv: Only unify 'riscv32/64' -> 'riscv' for host cpu in meson

We want to keep the ability to distinct between 32/64-bit host.

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: <20230627143235.29947-2-philmd@linaro.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>