runtime: make debug.FreeOSMemory call runtime.GC()

Currently freeOSMemory calls gcStart directly, but we really just want
it to behave like runtime.GC() and then perform a scavenge, so make it
call runtime.GC() rather than gcStart.

For #18216.

Change-Id: I548ec007afc788e87d383532a443a10d92105937
Reviewed-on: https://go-review.googlesource.com/37518
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mheap.go

@@ -1062,7 +1062,7 @@ func (h *mheap) scavenge(k int32, now, limit uint64) {
 
 //go:linkname runtime_debug_freeOSMemory runtime/debug.freeOSMemory
 func runtime_debug_freeOSMemory() {
-	gcStart(gcForceBlockMode, gcTrigger{kind: gcTriggerAlways})
+	GC()
 	systemstack(func() { mheap_.scavenge(-1, ^uint64(0), 0) })
 }