os/exec: remove flaky TestExtraFilesFDShuffle test

Update issue 5780

R=golang-dev, cshapiro, dave, bradfitz
CC=golang-dev
https://codereview.appspot.com/12869049
cmd/gc: Reset haspointers computation.  When converting from a
slice type to an array type, the haspointer-ness may change.
Before this change, we'd sometimes get types like [1]int marked
as having pointers.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/13189044