Weaken needlessly restrictive orderings on Arc::*_count

[SUPERCILEX]

Follow up to #95183 from this zulip: https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/Why.20does.20Arc.3A.3Astrong_count.20use.20Acquire.20instead.20of.20Relaxed.3F/near/386213850

I'd like to use the strong_count for a lockless algorithm I'm writing, but I don't need acquire semantics so that's pointlessly restrictive on arm/risc-v.

[rustbot]

r? @m-ou-se

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[m-ou-se]

@Amanieu Do you have an opinion on this?

[Amanieu]

I'm happy with this change. No guarantees are made in the API docs regarding memory ordering, so on that basis this is not a breaking change.

@bors r+

[bors]

📌 Commit 4c2f1c6 has been approved by Amanieu

It is now in the queue for this repository.

[bors]

⌛ Testing commit 4c2f1c6 with merge 4efd655...

[bors]

☀️ Test successful - checks-actions
Approved by: Amanieu
Pushing 4efd655 to master...

[rust-timer]

Finished benchmarking commit (4efd655): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.6%	[1.9%, 3.3%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.5%	[-3.0%, -2.1%]	5
All ❌✅ (primary)	-	-	0

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 631.081s -> 631.369s (0.05%)
Artifact size: 317.36 MiB -> 317.41 MiB (0.02%)