misc/dist: support building statically linked toolchain.

misc/dist: support building statically linked toolchain.
so that we don't need worry about specifying the required
libc version (note: as cmd/go will still be dynamically
linked to libc, we still need to perform the build on OSes
with an old enough libc. But as cmd/go doesn't rely on many
libc symbols, the situation should be significantly better).

Fixes #3564.

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/14261043