realm: dynamic load of gasnetex (stage 1)

[elliottslaughter]

This is a tracker issue for:

https://gitlab.com/StanfordLegion/legion/-/merge_requests/1229

So that I can make sure it gets into the June release.

Current status: needs testing on Frontier to make sure everything is working.

[elliottslaughter]

Merged via https://gitlab.com/StanfordLegion/legion/-/merge_requests/1427