Run benchmarks in CI on macOS in addition to linux

[kukushechkin]

Run benchmarks on macOS in addition to Linux.

Motivation:

1. macOS and Linux benchmark yield different results and we would like to rack both of them.
2. default Linux GitHub workers do not provide access to PMU, which is necessary for instructions metric benchmarking

Modifications:

• benchmarks.yml workflow support running benchmarks on self-hosted macOS runners

Result:

Benchmarks are running on both linux and macOS runners. An example of an opt-in PR running just one Xcode version: https://github.com/apple/swift-nio/actions/runs/19468002318/job/55707591945.

An example of an opt-in PR and thresholds expectations check failed:
https://github.com/apple/swift-nio/actions/runs/19468236909/job/55708359111

I've been running swift-nio benchmarks on macOS runners and it is much less stable than Linux for allocations and context switches. I suggest we make this an opt-in feature and take in only for instructions count benchmarks, for example upcoming to the swift-log. This also makes it less stressful for macOS runners pool.

[FranzBusch]

I know this is still a draft but I think we need to consider resource capacity here. The reason we haven't been running benchmarks on macOS is due to the limited number of available macOS runners. Let's explore if we can get custom Linux runners that provide access to the PMU.

[FranzBusch]

Overall this looks sensible to me since it allows user's to run benchmarks on macOS but those are disabled by default. Would love for @rnro to also take a look

[rnro]

Looks good, thanks!