target/riscv/cpu.c: add riscv_cpu_is_32bit()

target/riscv/cpu.c: add riscv_cpu_is_32bit()

Next patch will need to retrieve if a given RISCVCPU is 32 or 64 bit.
The existing helper riscv_is_32bit() (hw/riscv/boot.c) will always check
the first CPU of a given hart array, not any given CPU.

Create a helper to retrieve the info for any given CPU, not the first
CPU of the hart array. The helper is using the same 32 bit check that
riscv_cpu_satp_mode_finalize() was doing.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20231218125334.37184-23-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>