riscv/gdbstub: add V bit to priv reg

riscv/gdbstub: add V bit to priv reg

This adds virtualization mode (V bit) as bit(2) of register `priv`
per RiscV debug spec v1.0.0-rc4. Checked with gdb-multiarch v12.1.

Note that GDB may display `INVALID` tag for `priv` reg when V bit
is set, this doesn't affect actual access to the bit though.

Signed-off-by: Yanfeng Liu <yfliu2008@qq.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <tencent_1993B55C24DE7979BF34B200F78287002907@qq.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>