gdbstub: riscv: fix the fflags registers

gdbstub: riscv: fix the fflags registers

While debugging an application with GDB the following might happen:

(gdb) return
Make xxx return now? (y or n) y
Could not fetch register "fflags"; remote failure reply 'E14'

This is because riscv_gdb_get_fpu calls riscv_csrrw_debug with a wrong csr
number (8). It should use the csr_register_map in order to reach the
riscv_cpu_get_fflags callback.

Signed-off-by: KONRAD Frederic <frederic.konrad@adacore.com>
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>