syscall, cmd/go/internal/lockedfile/internal/filelock: add and use Flock on illumos

Copy the syscall wrapper from golang.org/x/sys/unix CL 255377 to provide
Flock on illumos and switch cmd/go/internal/lockedfile/internal/filelock
to use it.

Fixes #35618

Change-Id: I876a2b782329a988fa85361fb1ea58eb6f329af1
Reviewed-on: https://go-review.googlesource.com/c/go/+/255258


Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>