cmd/dist, build: support building statically linked toolchain

Added a new $GO_DISTFLAGS to make.bash, and while we're here,
added mention $CXX in make.bash (CL 13704044).

Fixes issue 6448.
Update issue 3564
We can pass GO_DISTFLAGS=-s from misc/dist to make.bash so that
it will build a statically linked toolchain.
(Note: OS X doesn't have the concept of static linking, so don't
pass GO_DISTFLAGS=-s for OS X builds)

R=adg, rsc, iant
CC=golang-dev
https://codereview.appspot.com/13887043