[release-branch.go1.1] runtime: introduce cnewarray() to simplify allocation of typed arrays

««« CL 9648044 / 139919984600
runtime: introduce cnewarray() to simplify allocation of typed arrays

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/9648044
»»»

R=dsymonds, dvyukov, dave
CC=golang-dev
https://golang.org/cl/9780050