runtime: only deduct assist credit for arenas during GC
CL 617876 has changed the original behavior. This modification will restore it.
Change-Id: I72cce82ebed362f99da7548035435397c835c99b
GitHub-Last-Rev:
980ef25b43cbae064f9c9cff0b589b2adbc72f1f
GitHub-Pull-Request: golang/go#74705
Reviewed-on: https://go-review.googlesource.com/c/go/+/689436
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Mark Freeman <mark@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>