fix: backport darwin/arm64 patches from main

This diff backports code to correctly handle darwin/arm64 from
`main` (see #7).

While there, this diff aims at reducing unnecessary differences with
the main branch, by removing code we also removed in there.

The reference issues are:

1. ooni/probe#2122

2. ooni/probe#2223

README.md

@@ -122,8 +122,8 @@ the package is only secure depending on the CPU configuration, which
 currently only holds for `aes` (see [aes/const.go](aes/const.go))
 
 - [ ] ensure that every forked package is never imported by using
-the following checks (we could also use `go list`, in principle, but
-I have not find a way for getting results for all architectures)
+the following checks (we could also use `go list` as follows
+`GOOS=os GOARCH=arch go list --json ./...`):
 
 1. `git grep 'subtle"'`
 

aes/cipher_asm.go

@@ -9,7 +9,7 @@ package aes
 import (
 	"crypto/cipher"
 
-	"github.com/ooni/oocrypto/internal/cpuarm64"
+	"github.com/ooni/oocrypto/internal/cpuoverlay"
 	"github.com/ooni/oocrypto/internal/subtle"
 	"golang.org/x/sys/cpu"
 )
@@ -29,8 +29,8 @@ type aesCipherAsm struct {
 	aesCipher
 }
 
-var supportsAES = cpu.X86.HasAES || cpuarm64.HasAES()
-var supportsGFMUL = cpu.X86.HasPCLMULQDQ || cpuarm64.HasPMULL()
+var supportsAES = cpu.X86.HasAES || cpuoverlay.Arm64HasAES()
+var supportsGFMUL = cpu.X86.HasPCLMULQDQ || cpuoverlay.Arm64HasPMULL()
 
 func newCipher(key []byte) (cipher.Block, error) {
 	if !supportsAES {

aes/cipher_generic.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 !amd64 && !s390x && !ppc64le && !arm64
+//go:build !amd64 && !arm64
 
 package aes