Cross compiled builds

Look what I made (git output redacted):

```bash
$ hkdist build all
build.go:153: git clone -b release https://github.com/kr/hk.git hk
...
build.go:153: git describe
build.go:121: GOOS=darwin GOARCH=386 go build -tags release -o hk
build.go:153: git clone -b release https://github.com/kr/hk.git hk
...
build.go:153: git describe
build.go:121: GOOS=darwin GOARCH=amd64 go build -tags release -o hk
build.go:153: git clone -b release https://github.com/kr/hk.git hk
...
build.go:153: git describe
build.go:121: GOOS=freebsd GOARCH=386 go build -tags release -o hk
build.go:153: git clone -b release https://github.com/kr/hk.git hk
...
build.go:153: git describe
build.go:121: GOOS=freebsd GOARCH=amd64 go build -tags release -o hk
build.go:153: git clone -b release https://github.com/kr/hk.git hk
...
build.go:153: git describe
build.go:121: GOOS=freebsd GOARCH=arm go build -tags release -o hk
build.go:153: git clone -b release https://github.com/kr/hk.git hk
...
build.go:153: git describe
build.go:121: GOOS=linux GOARCH=386 go build -tags release -o hk
build.go:153: git clone -b release https://github.com/kr/hk.git hk
...
build.go:153: git describe
build.go:121: GOOS=linux GOARCH=amd64 go build -tags release -o hk
build.go:153: git clone -b release https://github.com/kr/hk.git hk
...
build.go:153: git describe
build.go:121: GOOS=linux GOARCH=arm go build -tags release -o hk
build.go:153: git clone -b release https://github.com/kr/hk.git hk
...
build.go:153: git describe
build.go:121: GOOS=windows GOARCH=386 go build -tags release -o hk
build.go:153: git clone -b release https://github.com/kr/hk.git hk
...
build.go:153: git describe
build.go:121: GOOS=windows GOARCH=amd64 go build -tags release -o hk
```

It probably still needs some cleanup.

Of course, this requires that you have a cross-compiled Go build for each environment locally.

Do you think I should try doing this with `CGO_ENABLED=0` for consistency across platforms?