crypto/rand: use the getentropy syscall on OpenBSD

crypto/rand: use the getentropy syscall on OpenBSD

Go already supports Linux's getrandom, which is a slightly modified
version of getentropy.

getentropy was added in OpenBSD 5.6. All supported versions of OpenBSD
include it so, unlike with Linux and getrandom, we don't need to test
for its presence.

Fixes #13785.

Change-Id: Ib536b96675f257cd8c5de1e3a36165e15c9abac9
Reviewed-on: https://go-review.googlesource.com/18219
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>