Add a debugger field to CPURISCVState.  Add riscv_csrrw_debug function
to set it.  Disable mode checks when debugger field true.

Signed-off-by: Jim Wilson <jimw@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20190212230903.9215-1-jimw@sifive.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>