We currently build with cgo to enable the fast notify trigger.  But
dockercore/golang-cross has too old a toolchain to enable CGO for
darwin/arm64.  So for darwin/arm64 we disable CGO and use notify's
kqueue support instead.

- Tweak the Makefile variables to allow specifying the Go version,
  build tags, and CGO_ENABLED on a per os/arch basis.  Reorganize
  the variables on a per os/arch basis.
- Use github.com/randall77/makefat to build a multi-arch ("fat")
  binary for Darwin for those that want it.