cmd/go: add -json flag to go env

cmd/go: add -json flag to go env

"go env" prints Go environment information as a shell script format by
default but it's difficult for some tools (e.g. editor packages) to
interpret it.

The -json flag prints the environment in JSON format which
can be easily interpreted by a lot of tools.

$ go env -json
{
        "CC": "gcc",
        "CGO_CFLAGS": "-g -O2",
        "CGO_CPPFLAGS": "",
        "CGO_CXXFLAGS": "-g -O2",
        "CGO_ENABLED": "1",
        "CGO_FFLAGS": "-g -O2",
        "CGO_LDFLAGS": "-g -O2",
        "CXX": "g++",
        "GCCGO": "gccgo",
        "GOARCH": "amd64",
        "GOBIN": "/home/haya14busa/go/bin",
        "GOEXE": "",
        "GOGCCFLAGS": "-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build498013955=/tmp/go-build -gno-record-gcc-switches",
        "GOHOSTARCH": "amd64",
        "GOHOSTOS": "linux",
        "GOOS": "linux",
        "GOPATH": "/home/haya14busa",
        "GORACE": "",
        "GOROOT": "/home/haya14busa/src/go.googlesource.com/go",
        "GOTOOLDIR": "/home/haya14busa/src/go.googlesource.com/go/pkg/tool/linux_amd64",
        "PKG_CONFIG": "pkg-config"
}

Also, it supports arguments with -json flag.

$ go env -json GOROOT GOPATH GOBIN
{
        "GOBIN": "/home/haya14busa/go/bin",
        "GOPATH": "/home/haya14busa",
        "GOROOT": "/home/haya14busa/src/go.googlesource.com/go"
}

Fixes #12567

Change-Id: I75db3780f14a8ab8c7fa58cc3c9cc488ef7b66a1
Reviewed-on: https://go-review.googlesource.com/38757
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>