runtime: make next_gc be heap size to trigger GC at

In the STW collector, next_gc was both the heap size to trigger GC at
as well as the goal heap size.

Early in the concurrent collector's development, next_gc was the goal
heap size, but was also used as the heap size to trigger GC at. This
meant we always overshot the goal because of allocation during
concurrent GC.

Currently, next_gc is still the goal heap size, but we trigger
concurrent GC at 7/8*GOGC heap growth. This complicates
shouldtriggergc, but was necessary because of the incremental
maintenance of next_gc.

Now we simply compute next_gc for the next cycle during mark
termination. Hence, it's now easy to take the simpler route and
redefine next_gc as the heap size at which the next GC triggers. We
can directly compute this with the 7/8 backoff during mark termination
and shouldtriggergc can simply test if the live heap size has grown
over the next_gc trigger.

This will also simplify later changes once we start setting next_gc in
more sophisticated ways.

Change-Id: I872be4ae06b4f7a0d7f7967360a054bd36b90eea
Reviewed-on: https://go-review.googlesource.com/8420
Reviewed-by: Russ Cox <rsc@golang.org>

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