syscall: provide and use fcntlPtr for all BSD platforms

Provide fcntlPtr for all BSD platforms, then use this for FcntlFlock.
This reduces darwin and openbsd specific code, as well as avoiding
the use of an indirect system call on OpenBSD.

Updates #63900

Change-Id: I5c701f0d8413fab5477b9e21381395621d1fb6d0
Reviewed-on: https://go-review.googlesource.com/c/go/+/540018


Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Run-TryBot: Joel Sing <joel@sing.id.au>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Josh Rickmar <jrick@zettaport.com>