runtime: do not allocate on every time.Sleep

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/37162
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Austin Clements <austin@google.com>