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 #47812

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>