Configuration to allow compiling a arm64 musl version (on amd64 hardware)

I tested this by building the last version of node 8, 10, 12, 14 and 16 on amd64 hardware, and doing the basic steps in a frontend theme project (npm install, grunt watch, ...), and it works without issues.

This addresses #50

I updated the readme.md best effort, but feel free to update if needed