[release-branch.go1.17] crypto/rand, internal/syscall/unix: don't use getentropy on iOS

CL 302489 switched crypto/rand to use getentropy on darwin, however this
function is not available on iOS. Enable getentropy only on macOS and
disable it on iOS.

Fixes #47814

Change-Id: Ib7ba5d77346aee87904bb93d60cacc845f5c0089
Reviewed-on: https://go-review.googlesource.com/c/go/+/343609
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
(cherry picked from commit 7007431374)
Reviewed-on: https://go-review.googlesource.com/c/go/+/343989

src/crypto/rand/rand_getentropy.go

@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build darwin || openbsd
-// +build darwin openbsd
+//go:build (darwin && !ios) || openbsd
+// +build darwin,!ios openbsd
 
 package rand
 

src/internal/syscall/unix/getentropy_darwin.go

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build darwin && !ios
+// +build darwin,!ios
+
 package unix
 
 import (