cpu: roll back "use IsProcessorFeaturePresent to calculate ARM64 on windows"

Roll back CL 740880 (commit fc646e489fd944b6f77d327ab77f1a4bab81d5ad)
because it adds a golang.org/x/sys/windows import to the cpu package.
The cpu package is vendored into the main Go tree and the policy there
does not currently permit this dependency.

For golang/go#76791.
Fixes golang/go#78259.

Change-Id: I72860cdf98192f4cbd2b3398926203a752d8d0fe
Cq-Include-Trybots: luci.golang.try:x_sys-gotip-windows-arm64
Reviewed-on: https://go-review.googlesource.com/c/sys/+/757482
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
diff --git a/cpu/cpu_other_arm64.go b/cpu/cpu_other_arm64.go
index 6c7c5bf..53f814d 100644
--- a/cpu/cpu_other_arm64.go
+++ b/cpu/cpu_other_arm64.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build !darwin && !linux && !netbsd && !openbsd && !windows && arm64
+//go:build !darwin && !linux && !netbsd && !openbsd && arm64
 
 package cpu
 
diff --git a/cpu/cpu_windows_arm64.go b/cpu/cpu_windows_arm64.go
deleted file mode 100644
index d09e85a..0000000
--- a/cpu/cpu_windows_arm64.go
+++ /dev/null
@@ -1,42 +0,0 @@
-// 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 (
-	"golang.org/x/sys/windows"
-)
-
-func doinit() {
-	// set HasASIMD and HasFP to true as per
-	// https://learn.microsoft.com/en-us/cpp/build/arm64-windows-abi-conventions?view=msvc-170#base-requirements
-	//
-	// The ARM64 version of Windows always presupposes that it's running on an ARMv8 or later architecture.
-	// Both floating-point and NEON support are presumed to be present in hardware.
-	//
-	ARM64.HasASIMD = true
-	ARM64.HasFP = true
-
-	if windows.IsProcessorFeaturePresent(windows.PF_ARM_V8_CRYPTO_INSTRUCTIONS_AVAILABLE) {
-		ARM64.HasAES = true
-		ARM64.HasPMULL = true
-		ARM64.HasSHA1 = true
-		ARM64.HasSHA2 = true
-	}
-	ARM64.HasSHA3 = windows.IsProcessorFeaturePresent(windows.PF_ARM_SHA3_INSTRUCTIONS_AVAILABLE)
-	ARM64.HasCRC32 = windows.IsProcessorFeaturePresent(windows.PF_ARM_V8_CRC32_INSTRUCTIONS_AVAILABLE)
-	ARM64.HasSHA512 = windows.IsProcessorFeaturePresent(windows.PF_ARM_SHA512_INSTRUCTIONS_AVAILABLE)
-	ARM64.HasATOMICS = windows.IsProcessorFeaturePresent(windows.PF_ARM_V81_ATOMIC_INSTRUCTIONS_AVAILABLE)
-	if windows.IsProcessorFeaturePresent(windows.PF_ARM_V82_DP_INSTRUCTIONS_AVAILABLE) {
-		ARM64.HasASIMDDP = true
-		ARM64.HasASIMDRDM = true
-	}
-	if windows.IsProcessorFeaturePresent(windows.PF_ARM_V83_LRCPC_INSTRUCTIONS_AVAILABLE) {
-		ARM64.HasLRCPC = true
-		ARM64.HasSM3 = true
-	}
-	ARM64.HasSVE = windows.IsProcessorFeaturePresent(windows.PF_ARM_SVE_INSTRUCTIONS_AVAILABLE)
-	ARM64.HasSVE2 = windows.IsProcessorFeaturePresent(windows.PF_ARM_SVE2_INSTRUCTIONS_AVAILABLE)
-	ARM64.HasJSCVT = windows.IsProcessorFeaturePresent(windows.PF_ARM_V83_JSCVT_INSTRUCTIONS_AVAILABLE)
-}