runtime: do not allocate on every time.Sleep

It's common for some goroutines to loop calling time.Sleep.
Allocate once per goroutine, not every time.
This comes up in runtime/pprof's background reader.

Change-Id: I89d17dc7379dca266d2c9cd3aefc2382f5bdbade
Reviewed-on: https://go-review.googlesource.com/37162Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/proc.go

@@ -2330,6 +2330,7 @@ func goexit0(gp *g) {
 	gp.waitreason = ""
 	gp.param = nil
 	gp.labels = nil
+	gp.timer = nil
 
 	// Note that gp's stack scan is now "valid" because it has no
 	// stack.

src/runtime/runtime2.go

@@ -376,6 +376,7 @@ type g struct {
 	waiting        *sudog         // sudog structures this g is waiting on (that have a valid elem ptr); in lock order
 	cgoCtxt        []uintptr      // cgo traceback context
 	labels         unsafe.Pointer // profiler labels
+	timer          *timer         // cached timer for time.Sleep
 
 	// Per-G GC state
 

src/runtime/time.go

@@ -50,7 +50,12 @@ func timeSleep(ns int64) {
 		return
 	}
 
-	t := new(timer)
+	t := getg().timer
+	if t == nil {
+		t = new(timer)
+		getg().timer = t
+	}
+	*t = timer{}
 	t.when = nanotime() + ns
 	t.f = goroutineReady
 	t.arg = getg()