cpu: better defaults on darwin ARM64

[ncruces]

This is motivated by
wazero/wazero#2473
which needs to special case darwin for testing the availability of
HasATOMICS, although all M-series CPUs support it.

We could do sysctlbyname, like syscall_darwin_x86_gc.go, if that's
preferred:
https://developer.apple.com/documentation/kernel/1387446-sysctlbyname/determining_instruction_set_characteristics

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Patch Set 1: Auto-Submit+1 Code-Review+2 Commit-Queue+1

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Patch Set 1:

Dry run: CV is trying the patch.

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This PR is being closed because golang.org/cl/743320 has been merged.