target/riscv: Specify the XLEN for CPUs

target/riscv: Specify the XLEN for CPUs

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bin.meng@windriver.com>
Tested-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Palmer Dabbelt <palmerdabbelt@google.com>
Acked-by: Palmer Dabbelt <palmerdabbelt@google.com>
Message-id: c1da66affbb83ec4a2fbeb0194293bd24d65f5dc.1608142916.git.alistair.francis@wdc.com