runtime: replace manually managed G dequeues with a type

There are two manually managed G dequeues. Abstract these both into a
shared gQueue type. This also introduces a gList type, which we'll use
to replace several manually-managed G lists in follow-up CLs.

This makes the code more readable and maintainable. gcFlushBgCredit in
particular becomes much easier to follow. It also makes it easier to
introduce more G queues in the future. Finally, the gList type clearly
distinguishes between lists of Gs and individual Gs; currently both
are represented by a *g, which can easily lead to confusion and bugs.

Change-Id: Ic7798841b405d311fc8b6aa5a958ffa4c7993c6c
Reviewed-on: https://go-review.googlesource.com/129396
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/runtime2.go

@@ -574,8 +574,7 @@ type schedt struct {
 	nmspinning uint32 // See "Worker thread parking/unparking" comment in proc.go.
 
 	// Global runnable queue.
-	runqhead guintptr
-	runqtail guintptr
+	runq     gQueue
 	runqsize int32
 
 	// Global cache of dead G's.