cmd/5g, cmd/6g, cmd/8g: more nil ptr to large struct checks

R=r, ken, khr, daniel.morsing
CC=dsymonds, golang-dev, rickyz
https://codereview.appspot.com/8925043
cmd/vet: clean up the test
- clean up the notypes version of the test so it's quiet
- change the package in the buildtag test to avoid confusing godoc

R=golang-dev, minux.ma, bradfitz
CC=golang-dev
https://codereview.appspot.com/8671049
net: deflake raw IP protocol entry test on Windows

Update issue 5344

R=golang-dev, dave, r, alex.brainman
CC=golang-dev
https://codereview.appspot.com/8934043
runtime: prevent the GC from seeing the content of a frame in runfinq()

Fixes issue 5348.

R=golang-dev, dvyukov
CC=golang-dev
https://codereview.appspot.com/8954044