crypto/rand: use arc4random(3) on macOS and iOS

crypto/rand: use arc4random(3) on macOS and iOS

It's been good since Sierra: it never fails, it's faster, it's available
on iOS (see #47812), and it still handles forks and reseeding.

On a M2 with macOS 14.3.1:

              │   sec/op    │   sec/op     vs base                │
    Read/32-8   413.7n ± 3%   249.7n ± 3%  -39.65% (p=0.000 n=10)
    Read/4K-8   7.097µ ± 6%   1.261µ ± 2%  -82.24% (p=0.000 n=10)

              │     B/s      │      B/s       vs base                 │
    Read/32-8   73.76Mi ± 3%   122.25Mi ± 3%   +65.73% (p=0.000 n=10)
    Read/4K-8   550.5Mi ± 6%   3099.0Mi ± 2%  +462.99% (p=0.000 n=10)

arc4random(3) would be a good replacement for getentropy(2) on FreeBSD
and NetBSD as well, but we don't get as easy access to libc there.

Cq-Include-Trybots: luci.golang.try:gotip-darwin-amd64-longtest,gotip-darwin-amd64-nocgo,gotip-darwin-arm64_13,gotip-darwin-amd64_11,gotip-darwin-amd64_12,gotip-darwin-amd64_13,gotip-darwin-amd64_14
Change-Id: Ia76824853be92b4d1786e23592a1d2ef24d8907d
Reviewed-on: https://go-review.googlesource.com/c/go/+/569655
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
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Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Mauri de Souza Meneguzzo <mauri870@gmail.com>