runtime: clean up park messages

This changes "mark worker (idle)" to "GC worker (idle)" so it's more
clear to users that these goroutines are GC-related. It changes "GC
assist" to "GC assist wait" to make it clear that the assist is
blocked.

Change-Id: Iafbc0903c84f9250ff6bee14baac6fcd4ed5ef76
Reviewed-on: https://go-review.googlesource.com/16511Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mgc.go

@@ -1359,7 +1359,7 @@ func gcBgMarkWorker(p *p) {
 		gopark(func(g *g, mp unsafe.Pointer) bool {
 			releasem((*m)(mp))
 			return true
-		}, unsafe.Pointer(mp), "mark worker (idle)", traceEvGoBlock, 0)
+		}, unsafe.Pointer(mp), "GC worker (idle)", traceEvGoBlock, 0)
 
 		// Loop until the P dies and disassociates this
 		// worker. (The P may later be reused, in which case

src/runtime/mgcmark.go

@@ -482,7 +482,7 @@ retry:
 			goto retry
 		}
 		// Park for real.
-		goparkunlock(&work.assistQueue.lock, "GC assist", traceEvGoBlock, 2)
+		goparkunlock(&work.assistQueue.lock, "GC assist wait", traceEvGoBlock, 2)
 
 		// At this point either background GC has satisfied
 		// this G's assist debt, or the GC cycle is over.