runtime,syscall: implement and use syscalln on darwin

runtime,syscall: implement and use syscalln on darwin

All darwin syscall implementations can be consolidated into a
single syscalln function, as already happens on Windows.

This reduces duplication and allows moving some logic from
runtime to syscall.

Updates #699135

Cq-Include-Trybots: luci.golang.try:gotip-darwin-arm64-longtest,gotip-darwin-amd64-longtest,x_sys-gotip-darwin-arm64-longtest,x_sys-gotip-darwin-amd64-longtest
Change-Id: If5de80442b1d4a1123258401a3ae21695e7c8f6b
Reviewed-on: https://go-review.googlesource.com/c/go/+/699177
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>