This is a much more GCE specific change for hypervisor detection. Works on arm64 and amd64 VM types.

Well drat. Thanks for the example of hardware that does that. The AArch64 exception model doc I read suggested that wasn't the accepted practice and I was hoping to find something more general than a giant stack of magic strings in identify_hypervisor_smbios. I'll abandon this approach and come back with the patch to smbios_subr.c after I've had a chance to test it.

I don't think moving vfp_save_state(td) is sufficient because of lazy SVE restoration. If the thread hasn't executed SVE instructions since a context switch, the trap to re-enable the unit won't have fired. In that state, vfp_save_state(td) sees the unit as disabled and won't set PCB_FP_SVEVALID and we'll still potentially see this panic. Checking pcb_svesaved != NULL avoids this by checking the backing store directly.