build: Unify build/crossbuild

Changes:

- allows explicit crossbuild targets (eg: `make mgmt-darwin-amd64`)
- adds darwin/amd64 to default crossbuild targets
- gitignore only build artifacts (eg: not all files starting with `mgmt-`)
- `build` and `crossbuild` target now utilize the same build function (`build` still generates only a `mgmt` binary for the current os/arch)
- test crossbuilding
- allow specifying custom GOOSARCHES envvar to override defaults
- crossbuild artifacts go into `build/` now
- add `build-debug` which includes symbol tables and debug info
- the build function now has `-s -w` linker arguments which discards some debug info afaict, to build a debug release use `make build-debug`

On my mac crossbuilding won't work unless I disable augeas and libvirt:

```
~/.g/s/g/p/mgmt (build|●1✚8…3) $ make build
Generating: bindata...
Generating: lang...
/Applications/Xcode.app/Contents/Developer/usr/bin/make --quiet -C lang
Building: mgmt, os/arch: darwin-amd64, version: 0.0.14-12-g94c8bc1-dirty...
env GOOS=darwin GOARCH=amd64 time go build -ldflags "-X main.program=mgmt -X main.version=0.0.14-12-g94c8bc1-dirty -s -w" -o mgmt-darwin-amd64 ;
        7.14 real        10.36 user         1.73 sys
mv mgmt-darwin-amd64 mgmt
```

```
~/.g/s/g/p/mgmt (build|●1✚8…3) $ time env GOTAGS='noaugeas novirt' make crossbuild
Generating: bindata...
Generating: lang...
/Applications/Xcode.app/Contents/Developer/usr/bin/make --quiet -C lang
Building: mgmt, os/arch: linux-amd64, version: 0.0.14-12-g94c8bc1-dirty...
env GOOS=linux GOARCH=amd64 time go build -ldflags "-X main.program=mgmt -X main.version=0.0.14-12-g94c8bc1-dirty -s -w" -o mgmt-linux-amd64 -tags 'noaugeas novirt';
       18.48 real        50.02 user         5.83 sys
Building: mgmt, os/arch: linux-ppc64, version: 0.0.14-12-g94c8bc1-dirty...
env GOOS=linux GOARCH=ppc64 time go build -ldflags "-X main.program=mgmt -X main.version=0.0.14-12-g94c8bc1-dirty -s -w" -o mgmt-linux-ppc64 -tags 'noaugeas novirt';
       29.83 real        85.09 user        11.54 sys
Building: mgmt, os/arch: linux-ppc64le, version: 0.0.14-12-g94c8bc1-dirty...
env GOOS=linux GOARCH=ppc64le time go build -ldflags "-X main.program=mgmt -X main.version=0.0.14-12-g94c8bc1-dirty -s -w" -o mgmt-linux-ppc64le -tags 'noaugeas novirt';
       29.74 real        85.84 user        11.76 sys
Building: mgmt, os/arch: linux-arm64, version: 0.0.14-12-g94c8bc1-dirty...
env GOOS=linux GOARCH=arm64 time go build -ldflags "-X main.program=mgmt -X main.version=0.0.14-12-g94c8bc1-dirty -s -w" -o mgmt-linux-arm64 -tags 'noaugeas novirt';
       28.33 real        83.24 user        11.40 sys
Building: mgmt, os/arch: darwin-amd64, version: 0.0.14-12-g94c8bc1-dirty...
env GOOS=darwin GOARCH=amd64 time go build -ldflags "-X main.program=mgmt -X main.version=0.0.14-12-g94c8bc1-dirty -s -w" -o mgmt-darwin-amd64 -tags 'noaugeas novirt';
        7.16 real        10.15 user         1.74 sys
      114.71 real       315.26 user        42.44 sys
```

.gitignore

@@ -9,7 +9,8 @@ tmp/
 bindata/*.go
 mgmt
 mgmt.static
-mgmt-*
+# crossbuild artifacts
+build/mgmt-*
 mgmt.iml
 rpmbuild/
 *.deb