runtime: set HeapGoal to zero when the GC is disabled

runtime: set HeapGoal to zero when the GC is disabled

When the GC is disabled, the tracer should emit a heap goal of 0. Not
setting the heap goal to 0 causes an inaccurate NextGC value to be
emmited.

Fixes #63864

Change-Id: Iecceaca86c0a43c1cc4d9433f1f9bb736f01ccbc
Reviewed-on: https://go-review.googlesource.com/c/go/+/639417
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>