Change the macOS implementation to use libc calls.
Using libc calls directly is what we do for all the runtime and os syscalls.
Doing so here as well improves consistency and also makes it possible
to cross-compile (from non-Mac systems) macOS binaries that use the
native name resolver.
Fixes #12524.
Change-Id: I011f4fcc5c50fbb5396e494889765dcbb9342336
Reviewed-on: https://go-review.googlesource.com/c/go/+/446178
Run-TryBot: Russ Cox <rsc@golang.org> Reviewed-by: Ian Lance Taylor <iant@google.com>