net: enable test for runtime.PollDesc leak

Update issue 5219.

R=golang-dev, dvyukov, r
CC=golang-dev
https://codereview.appspot.com/8602047
cmd/ld: do not relocate ignored PE sections

This should have been part of CL8715043.

Update issue 5106

R=golang-dev, r, iant
CC=golang-dev
https://codereview.appspot.com/8676050