[cdac] Always re-read global pointers in GetUsefulGlobals

[elinor-fung]

GetUsefulGlobals can be called before the runtime is actually initialized, so we need to re-read the global pointers instead of storing any of them.

Contributes to #99302

[dotnet-policy-service]

Tagging subscribers to this area: @tommcdon
See info in area-owners.md if you want to be subscribed.