offload: move (un)register lib into global_ctors

[ZuseZ4]

Right now we initialize the openmp/offload runtime before every single offload call, and tear it down directly afterwards.
What we should rather do is initialize it once in the binary startup code, and tear it down at the end of the binary execution. Here I implement these changes.

Together, our generated IR has a lot less usage of globals, which in turn simplifies the refactoring in #150683, where I introduce a new variant of our offload intrinsic.

r? oli-obk

[rustbot]

oli-obk is not on the review rotation at the moment.
They may take a while to respond.

[ZuseZ4]

@kevinsala #150683 fails with the commit where I drop init_all_rtls(), it also mentions that 0 devices are available.

omptarget fatal error -1: "invalid value" device number '-1' out of range, only 0 devices available

[ZuseZ4]

I dropped the last commit, since we'll still need init_all_rtls till I replace the clang-linker-wrapper. I left a note instead.
@rustbot ready

[ZuseZ4]

Since oli is currently busy,
@rustbot reroll

[chenyukang]

@rustbot reroll

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.