cmd/go: print more env variables in "go env"

"go env" previously only printed a subset of the documented environment
variables; now it includes everything, such as GO386 and CGO_*.

This also fixes the CGO_CFLAGS environment variable to always have the
same default. According to iant@ and confirmed by testing, cgo can now
understand the default value of CGO_CFLAGS.

Fixes #17191.

Change-Id: Icf75055446dd250b6256ef1139e9ce848f4a9d3b
Reviewed-on: https://go-review.googlesource.com/31330


TryBot-Result: Gobot Gobot <gobot@golang.org>
Run-TryBot: Quentin Smith <quentin@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>