internal/poll, internal/syscall/unix, net, runtime: convert openbsd (except mips64...

internal/poll, internal/syscall/unix, net, runtime: convert openbsd (except mips64) to direct libc calls

Call libc wrappers directly rather than calling using syscall(2).

Updates golang/go#36435

Change-Id: I40be410c7472f7d89cbec2ebdc7c841c7726ca4a
Reviewed-on: https://go-review.googlesource.com/c/go/+/425637
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Joel Sing <joel@sing.id.au>