crypto/rand: use arc4random_buf() on OpenBSD

crypto/rand: use arc4random_buf() on OpenBSD

OpenBSD system calls are mediated by libc anyway, and arc4random_buf()
is the preferred mechanism to obtain random bytes.

Also, rename NetBSD's function to reflect it's not actually calling
getentropy(3).

Cq-Include-Trybots: luci.golang.try:gotip-openbsd-amd64
Change-Id: Id1f3f7af16750537e2420bcf44b086de5854198c
Reviewed-on: https://go-review.googlesource.com/c/go/+/608395
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Bypass: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>