to reduce compile time memory/stack usage.
Update #4970
$ go test -c ../test/bench/go1
before:
0.36user 0.07system 0:00.44elapsed 100%CPU
(0avgtext+0avgdata 540720maxresident)k
0inputs+19840outputs (0major+56451minor)pagefaults 0swaps
after:
0.33user 0.05system 0:00.39elapsed 100%CPU
(0avgtext+0avgdata 289936maxresident)k
0inputs+19864outputs (0major+29615minor)pagefaults 0swaps

And stack usage is reduced to below 1MiB.

R=golang-dev, r, dave
CC=golang-dev
https://golang.org/cl/7436050