go/ssa/interp: add intrinsic for runtime.environ, recently added.
go/ssa/interp: add intrinsic for runtime.environ, recently added.

LGTM=gri
R=gri
CC=golang-codereviews
https://codereview.appspot.com/135120043

karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools