runtime: check transition condition before triggering periodic GC

Currently sysmon triggers periodic GC if GC is not currently running
and it's been long enough since the last GC. This misses some
important conditions; for example, whether GC is enabled at all by
GOGC. As a result, if GOGC is off, once we pass the timeout for
periodic GC, sysmon will attempt to trigger a GC every 10ms. This GC
will be a no-op because gcStart will check all of the appropriate
conditions and do nothing, but it still goes through the motions of
waking the forcegc goroutine and printing a gctrace line.

Fix this by making sysmon call gcShouldStart to check *all* of the
appropriate transition conditions before attempting to trigger a
periodic GC.

Fixes #19247.

Change-Id: Icee5521ce175e8419f934723849853d53773af31
Reviewed-on: https://go-review.googlesource.com/37515
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/proc.go

@@ -3791,7 +3791,7 @@ func sysmon() {
 		}
 		// check if we need to force a GC
 		lastgc := int64(atomic.Load64(&memstats.last_gc_nanotime))
-		if gcphase == _GCoff && lastgc != 0 && now-lastgc > forcegcperiod && atomic.Load(&forcegc.idle) != 0 {
+		if gcShouldStart(true) && lastgc != 0 && now-lastgc > forcegcperiod && atomic.Load(&forcegc.idle) != 0 {
 			lock(&forcegc.lock)
 			forcegc.idle = 0
 			forcegc.g.schedlink = 0