mac: Add macOS x86 support

[pingleig]

Description of the issue

Support packaging macOS x86 binary.

Description of changes

• Split mac and linux logic, previously we pretend mac is linux and claim all the metrics are supported, which is not the case e.g. procstat. Now the agent will fail to start on mac if there are unsupported metrics in config instead of skipping it silently at runtime.
• Package macOS binary as a tar (i.e. not dmg nor pkg due to signing), there will be official pkg release.
• Fix build: The date --iso-8601=seconds option does not work on macOS #101 make should no longer has error on date flags.
• Support run as user on macOS, implemented a non cgo user lookup logic because of os/user: LookupUser() doesn't find users on macOS when compiled with CGO_ENABLED=0 golang/go#24383 (comment) (btw: might need to port it to otel as well Run the collector as a custom user  aws-observability/aws-otel-collector#203)

License

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Tests

make build
make test