runtime/_mkmalloc: add a copy of cloneNode

runtime/_mkmalloc: add a copy of cloneNode

cloneNode is defined in golang.org/x/tools/internal/astutil. Make a copy
of it so we can easily clone AST nodes.

Change-Id: I6a6a6964132e663e64faa00fa6037cf6e8d4cbc6
Reviewed-on: https://go-review.googlesource.com/c/go/+/703396
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Matloob <matloob@google.com>