Support cross compilation.

[Doron Behar]

So artifacts can be published in https://github.com/doronbehar/pistol/releases for many systems. There's some help at:

https://nix.dev/tutorials/cross-compilation#cross-compilation

But I get now the error:

pistol-static-aarch64-unknown-linux-musl> aarch64-unknown-linux-musl-gcc: error: unrecognized command line option '-m64'