[dev.typeparams] runtime/internal/sys: replace GOOS with goos.GOOS

[dev.typeparams] runtime/internal/sys: replace GOOS with goos.GOOS

Refactoring done by rf tool:

    rf 'ex . {
	import "internal/goos"
	import "runtime/internal/sys"
	sys.GOOS -> goos.GOOS
    }'

Change-Id: I4b4aadff8640731ce4cb9bdad9954c267eb484c8
Reviewed-on: https://go-review.googlesource.com/c/go/+/328339
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>