target/riscv: Expose "priv" register for GDB for reads

This patch enables a debugger to read the current privilege level via a virtual
"priv" register. When compiled with CONFIG_USER_ONLY the register is still
visible but always reports the value zero.

Signed-off-by: Jonathan Behrens <jonathan@fintelia.io>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>