[release-branch.go1.1] runtime: fix GC scanning of slices

««« CL 9372044 / 1abed5873071
runtime: fix GC scanning of slices
If a slice points to an array embedded in a struct,
the whole struct can be incorrectly scanned as the slice buffer.
Fixes #5443.

R=cshapiro, iant, r, cshapiro, minux.ma
CC=bradfitz, gobot, golang-dev
https://golang.org/cl/9372044
»»»

R=cshapiro, iant
CC=golang-dev
https://golang.org/cl/10027043