Comment 2 for bug 1836192

We didn't spot that armv5 CPUs don't have mvfr0, so now the vfp refactor is looking at mvfr0 fields to gate feature presence we need to initialize cpu->isar.mvfr0 specifically to a value that indicates the right thing even on the armv5 CPUs which don't have a guest-visible mvfr0. This specifically affects just arm926 and arm1026, which have accidentally lost short-vector support and double-precision support.