os/signal/internal/pty: use libc (not cgo) on macOS
This package is only used by tests anyway, but might as well
remove the cgo use on macOS so that it doesn't show up as
a cgo user, as part of our overall strategy to remove cgo use
in the standard library on macOS.
Change-Id: I5a1a39ed56373385f9d43a5e17098035dc1a451a
Reviewed-on: https://go-review.googlesource.com/c/go/+/449315
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org> Reviewed-by: Cherry Mui <cherryyz@google.com> Reviewed-by: Ian Lance Taylor <iant@google.com>