Hermit: Unify `std::env::args` with Unix
The only differences between these implementations of `std::env::args` are that Unix uses relaxed ordering, but Hermit uses acquire/release, and Unix truncates `argv` at the first null pointer, but Hermit doesn't. Since Hermit aims for Unix compatibility, unify it with Unix.
The atomic orderings were established in https://github.com/rust-lang/rust/pull/74006 (cc `@euclio)` for Unix and https://github.com/rust-lang/rust/pull/100579 (cc `@joboet)` for Hermit and, before those, they used mutexes and non-atomic statics. I think the difference in orderings is simply from them being changed at different times. The commented explanation for using acquire/release for Hermit is “to broadcast writes by the OS”. I'm not experienced enough with atomics to accurately judge, but I think acquire/release is stronger than needed. Either way, they should match.
Truncating at the first null pointer seems desirable, though I don't know whether it is necessary in practice on Hermit.
cc `@mkroening` `@stlankes` for Hermit
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