cpu: read ARM64 registers using sysctl(3) on netbsd/arm64

Use sysctl("machdep.cpu0.cpu_id") to retrieve the ARM64 cpu_id
information on netbsd/arm64. This includes the ISAR0, ISAR1 and PFR0
registers which can be parsed using existing functionality.

In order to use sysctl(3) without depending on x/sys/unix, some
functionality needed to be copied and slightly adjusted.

Change-Id: Ic9a133917ae5da516e6794f375bd7602d9bd3aa6
Reviewed-on: https://go-review.googlesource.com/c/sys/+/264378
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Trust: Benny Siegert <bsiegert@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>

cpu/cpu_arm64.go

@@ -39,28 +39,26 @@ func initOptions() {
 
 func archInit() {
 	switch runtime.GOOS {
-	case "android", "darwin", "ios", "netbsd", "openbsd":
-		// Android and iOS don't seem to allow reading these registers.
-		//
-		// NetBSD:
-		// ID_AA64ISAR0_EL1 is a privileged register and cannot be read from EL0.
-		// It can be read via sysctl(3). Example for future implementers:
-		// https://nxr.netbsd.org/xref/src/usr.sbin/cpuctl/arch/aarch64.c
+	case "freebsd":
+		readARM64Registers()
+	case "linux", "netbsd":
+		doinit()
+	default:
+		// Most platforms don't seem to allow reading these registers.
 		//
 		// OpenBSD:
 		// See https://golang.org/issue/31746
-		//
-		// Fake the minimal features expected by
-		// TestARM64minimalFeatures.
-		ARM64.HasASIMD = true
-		ARM64.HasFP = true
-	case "linux":
-		doinit()
-	default:
-		readARM64Registers()
+		setMinimalFeatures()
 	}
 }
 
+// setMinimalFeatures fakes the minimal ARM64 features expected by
+// TestARM64minimalFeatures.
+func setMinimalFeatures() {
+	ARM64.HasASIMD = true
+	ARM64.HasFP = true
+}
+
 func readARM64Registers() {
 	Initialized = true