Currently, the computations that derive controls from the GC trigger
are spread across several parts of the mark termination code.
Consolidate computing the absolute trigger, the heap goal, and sweep
pacing into a single function called at the end of mark termination.

Unlike the code being consolidated, this has to be more careful about
negative gcpercent. Many of the consolidated code paths simply didn't
execute if GC was off.

This is a step toward being able to change the GC trigger ratio in the
middle of concurrent sweeping and marking. For this commit, we try to
stick close to the original structure of the code that's being
consolidated, so it doesn't yet support mid-cycle adjustments.

For #19076.

Change-Id: Ic5335be04b96ad20e70d53d67913a86bd6b31456
Reviewed-on: https://go-review.googlesource.com/39831
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>