Check soundness of static ref inside `SystemTable`

[nicholasbishop]

It was pointed out in a reddit thread that this might not be sound when exiting boot services: https://www.reddit.com/r/rust/comments/11m3bq8/comment/jbid9sd/?utm_source=reddit&utm_medium=web2x&context=3 Thanks @DianaNites!

I haven't taken a proper look at it yet, but filing an issue as a reminder.

[phil-opp]

Now that #710 is merged, can this be closed?

[nicholasbishop]

Yep this is done. I have some more investigation I want to do regarding potential UB, tracked in #715.