config: Default the indirect branch predictor to "None".

config: Default the indirect branch predictor to "None".

Other scripts (like se.py) blindly try to apply the indirect predictor
if one is set. Because this option defaults to something, there's no
way (as far as I know) to purposefully select nothing, and so the
simulator crashes. Users shouldn't have to proactively prevent gem5
from killing itself regardless, so the default was changed to "None".

Change-Id: Ic3382b8065442d6705b1c6a656646598d9d5c322
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23360
Reviewed-by: Ciro Santilli <ciro.santilli@arm.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>