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_darwin_arm64.go

@@ -0,0 +1,26 @@
+// Copyright 2026 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cpu
+
+import "runtime"
+
+func doinit() {
+	ARM64.HasASIMD = true
+	ARM64.HasFP = true
+
+	if runtime.GOOS != "ios" {
+		// M-series SoCs are at least armv8.4-a
+		ARM64.HasCRC32 = true   // armv8.1
+		ARM64.HasATOMICS = true // armv8.2
+		ARM64.HasJSCVT = true   // armv8.3, if HasFP
+
+		// Go already assumes these to be available
+		// because they were on the M1.
+		ARM64.HasAES = true
+		ARM64.HasPMULL = true
+		ARM64.HasSHA1 = true
+		ARM64.HasSHA2 = true
+	}
+}