runtime: don't adjust GC trigger on forced GC

Forced GCs don't provide good information about how to adjust the GC
trigger. Currently we avoid adjusting the trigger on forced GC because
forced GC is also STW and we don't adjust the trigger on STW GC.
However, this will become a problem when forced GC is concurrent.

Fix this by skipping trigger adjustment if the GC was user-forced.

For #18216.

Change-Id: I03dfdad12ecd3cfeca4573140a0768abb29aac5e
Reviewed-on: https://go-review.googlesource.com/38951
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mgc.go

@@ -552,6 +552,13 @@ func (c *gcControllerState) revise() {
 // endCycle updates the GC controller state at the end of the
 // concurrent part of the GC cycle.
 func (c *gcControllerState) endCycle() {
+	if work.userForced {
+		// Forced GC means this cycle didn't start at the
+		// trigger, so where it finished isn't good
+		// information about how to adjust the trigger.
+		return
+	}
+
 	h_t := c.triggerRatio // For debugging
 
 	// Proportional response gain for the trigger controller. Must