runtime: eliminate runqvictims and a copy from runqsteal

Currently, runqsteal steals Gs from another P into an intermediate
buffer and then copies those Gs into the current P's run queue. This
intermediate buffer itself was moved from the stack to the P in commit
c4fe503 to eliminate the cost of zeroing it on every steal.

This commit follows up c4fe503 by stealing directly into the current
P's run queue, which eliminates the copy and the need for the
intermediate buffer. The update to the tail pointer is only committed
once the entire steal operation has succeeded, so the semantics of
stealing do not change.

Change-Id: Icdd7a0eb82668980bf42c0154b51eef6419fdd51
Reviewed-on: https://go-review.googlesource.com/9998
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Austin Clements <austin@google.com>

src/runtime/runtime2.go

@@ -353,10 +353,9 @@ type p struct {
 	goidcacheend uint64
 
 	// Queue of runnable goroutines. Accessed without lock.
-	runqhead    uint32
-	runqtail    uint32
-	runq        [256]*g
-	runqvictims [128]*g // Used to stage victims from another p's runq
+	runqhead uint32
+	runqtail uint32
+	runq     [256]*g
 	// runnext, if non-nil, is a runnable G that was ready'd by
 	// the current G and should be run next instead of what's in
 	// runq if there's time remaining in the running G's time