runtime: add garbage collector statistics

If the constant CollectStats is non-zero and GOGCTRACE=1
the garbage collector will print basic statistics about executed
GC instructions.

R=golang-dev, dvyukov
CC=golang-dev, rsc
https://codereview.appspot.com/7413049
cmd/gc: disallow selectors to the blank identifier

Fixes issue 4941.

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/7415051