Introduce 'all' build tag for linting

[silverwind]

By introducing a new 'all' build tag on all build conditionals, we can make the linters run on all files, eliminating two costly steps on each CI run. Thanks to golangci/golangci-lint#1646 (comment) for the idea.

[silverwind]

Added a check for this constraint as well, in case of error, it prints:

Please include the 'all' tag in all +build constraints, e.g. '// +build all yourtag'
build/generate-emoji.go:// +build ignore
make: *** [Makefile:277: build-constraints-check] Error 1

[silverwind]

Put on WIP because it currently crashes with runtime: out of memory with the--build-tags all option. I'm pretty convinced that the previous version of golangci-lint linting did not honor the passed TAGS at all which I think means that all files with build tags are currently not being linted at all.

[silverwind]

golangci took 33 minutes for whatever reason, I don't think this is going to work out as planned.