darwin/arm64 support for internal/cpu was implemented in c155931 using a mix of dynamic detection with sysctlbyname and hardcoded minimal feature support assumed on darwin/arm64.

For x/sys/cpu it can be assumed it only needs to work with Go 1.16 and newer. Interestingly darwing/arm64 was previosly used for the now ios/arm64 port. We should keep compatibility such that old code still compiles pre Go 1.16 even if there may be no runtime detection supported.

Implementing support for x/sys/cpu will be a bit more complex. While x/sys/cpu does not need to be as low dependency as internal/cpu it should still avoid uncommon or large dependencies. Note that x/sys/cpu is vendored into the standard library for some crypto support that is also vendored in.

x/sys/cpu should avoid a dependency on x/sys/unix: #32102