archive/tar: make test pass on setgid dirs

Fixes issue 4867

R=golang-dev, dsymonds
CC=golang-dev
https://codereview.appspot.com/7382045
cmd/gc, reflect, runtime: switch to indirect func value representation

Step 1 of http://golang.org/s/go11func.

R=golang-dev, r, daniel.morsing, remyoudompheng
CC=golang-dev
https://codereview.appspot.com/7393045