target/riscv: correct "code should not be reached" for x-rv128

target/riscv: correct "code should not be reached" for x-rv128

The addition of uxl support in gdbstub adds a few checks on the maximum
register length, but omitted MXL_RV128, an experimental feature.
This patch makes rv128 react as rv64, as previously.

Signed-off-by: Frédéric Pétrot <frederic.petrot@univ-grenoble-alpes.fr>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: LIU Zhiwei <zhiwei_liu@c-sky.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20220124202456.420258-1-frederic.petrot@univ-grenoble-alpes.fr
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>