runtime: make sysmon-triggered GC concurrent

sysmon triggers a GC if there has been no GC for two minutes.
Currently, this is a STW GC. There is no reason for this to be STW, so
make it concurrent.

Fixes #10261.

Change-Id: I92f3ac37272d5c2a31480ff1fa897ebad08775a9
Reviewed-on: https://go-review.googlesource.com/11955Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/proc.go

@@ -154,7 +154,7 @@ func forcegchelper() {
 		if debug.gctrace > 0 {
 			println("GC forced")
 		}
-		startGC(gcForceMode)
+		startGC(gcBackgroundMode)
 	}
 }
 

src/runtime/proc1.go

@@ -3004,7 +3004,7 @@ func sysmon() {
 		}
 		// check if we need to force a GC
 		lastgc := int64(atomicload64(&memstats.last_gc))
-		if lastgc != 0 && unixnow-lastgc > forcegcperiod && atomicload(&forcegc.idle) != 0 {
+		if lastgc != 0 && unixnow-lastgc > forcegcperiod && atomicload(&forcegc.idle) != 0 && atomicloaduint(&bggc.working) == 0 {
 			lock(&forcegc.lock)
 			forcegc.idle = 0
 			forcegc.g.schedlink = 0