cmd/vet: check return moves, support power64x, various fixes
cmd/vet: check return moves, support power64x, various fixes

This adds support for checking moves to the return value stack
slot (from rsc), adds support for checking power64x assembly,
fixes argument offset checking and leaf function support on
platforms with a link register (arm and power64).

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/166920043

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