Missing native binaries for darwin/arm64

[rozcietrzewiacz]

On Mac M1, the x86_64 binary of gbt works through compatibility wrapping, which makes it noticeably slower. Here's a timing comparison:

• upstream version (obtained through brew install)

$ time gbt &>/dev/null

real	0m0.174s
user	0m0.081s
sys	0m0.084s

• Compiled native arm64 version:

$ time ./gbt &>/dev/null

real	0m0.094s
user	0m0.052s
sys	0m0.041s

The (almost) 100ms increase creates a noticeable and annoying latency.

[jtyr]

I will build native version from darwin/arm64 once I get to cut a new release. I just need rewrite all TravisCI automation into GitHub Workflows.