std: use OnceLock for SGX argument storage

[joboet]

Just like #158180, using a OnceLock is much better. Though I guess that in this case a static mut would be fine as well...

CC @jethrogb @raoulstrackx @aditijannu

[rustbot]

r? @clarfonthey

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[clarfonthey]

Looks reasonable, r=me if the SGX folks can also sign off on this.

[rustbot]

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

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