runtime: split gfree list into with-stacks and without-stacks

Currently all free Gs are added to one list. Split this into two
lists: one for free Gs with cached stacks and one for Gs without
cached stacks.

This lets us preferentially allocate Gs that already have a stack, but
more importantly, it sets us up to free cached G stacks concurrently.

Change-Id: Idbe486f708997e1c9d166662995283f02d1eeb3c
Reviewed-on: https://go-review.googlesource.com/20664
Reviewed-by: Rick Hudson <rlh@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/runtime2.go

@@ -523,9 +523,10 @@ type schedt struct {
 	runqsize int32
 
 	// Global cache of dead G's.
-	gflock mutex
-	gfree  *g
-	ngfree int32
+	gflock       mutex
+	gfreeStack   *g
+	gfreeNoStack *g
+	ngfree       int32
 
 	// Central cache of sudog structs.
 	sudoglock  mutex