cpu: better defaults on darwin ARM64

This is motivated by
https://github.com/wazero/wazero/pull/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

Change-Id: I0f5ea55ef5cda4956db44b2d40f2f10044fa3321
GitHub-Last-Rev: d5d0f1e989579de2a25c97c00c7e9db5d4b0cb29
GitHub-Pull-Request: golang/sys#267
Reviewed-on: https://go-review.googlesource.com/c/sys/+/743320
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>

cpu/cpu_arm64.go

@@ -44,14 +44,11 @@ func initOptions() {
 }
 
 func archInit() {
-	switch runtime.GOOS {
-	case "freebsd":
+	if runtime.GOOS == "freebsd" {
 		readARM64Registers()
-	case "linux", "netbsd", "openbsd", "windows":
+	} else {
+		// Most platforms don't seem to allow directly reading these registers.
 		doinit()
-	default:
-		// Many platforms don't seem to allow reading these registers.
-		setMinimalFeatures()
 	}
 }