This slightly rearranges gcSetTriggerRatio to compute the goal before
computing the other controls. This will simplify implementing the heap
limit, which needs to control the absolute goal and flow the rest of
the control parameters from this.

For #16843.

Change-Id: I46b7c1f8b6e4edbee78930fb093b60bd1a03d75e
Reviewed-on: https://go-review.googlesource.com/c/go/+/46750
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Rick Hudson <rlh@golang.org>