[dev.typeparams] internal/goarch,internal/goos: explode runtime/internal/sys into pieces

This change extracts the GOOS and GOARCH specific constants from
runtime/internal/sys into packages that are available to the entire
standard library.

This change does not yet update the runtime and associated packages to
use them, and instead adds constants to runtime/internal/sys to forward
the constants defined by these new packages.

Change-Id: I14d574b8d7bfe599ad25da29dc1b39716e35a734
Reviewed-on: https://go-review.googlesource.com/c/go/+/328336


Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>