cmd/go: add -p flag for parallelism (like make -j)

cmd/go: add -p flag for parallelism (like make -j)

On my MacBookAir4,1:

19.94r 	 go install -a -p 1 std
12.36r 	 go install -a -p 2 std
9.76r 	 go install -a -p 3 std
10.77r 	 go install -a -p 4 std

86.57r 	 go test -p 1 std -short
52.69r 	 go test -p 2 std -short
43.75r 	 go test -p 3 std -short
40.44r 	 go test -p 4 std -short

157.50r 	 go test -p 1 std
99.58r 	 go test -p 2 std
87.24r 	 go test -p 3 std
80.18r 	 go test -p 4 std

R=golang-dev, adg, r
CC=golang-dev
https://golang.org/cl/5531057