hw/riscv: Use the CPU to determine if 32-bit

hw/riscv: Use the CPU to determine if 32-bit

Instead of using string compares to determine if a RISC-V machine is
using 32-bit or 64-bit CPUs we can use the initalised CPUs. This avoids
us having to maintain a list of CPU names to compare against.

This commit also fixes the name of the function to match the
riscv_cpu_is_32bit() function.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 8ab7614e5df93ab5267788b73dcd75f9f5615e82.1608142916.git.alistair.francis@wdc.com