cmd/compile: use slices.SortStableFunc

cmd/compile: use slices.SortStableFunc

Now that we're bootstrapping from a toolchain that has the slices
package.

Updates #64751

Change-Id: I876ec6d261466344faf33f8c5cda229dd1e4185f
Reviewed-on: https://go-review.googlesource.com/c/go/+/610602
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>