Improve cross-compile support

[clangen]

Summary of changes:

• Add support for armv6 builds, so musikcube can run on Raspberry Pi Zero devices
• Switched from Debian's arm cross-compile toolchain to a more actively maintained one here: https://github.com/tttapa/docker-arm-cross-toolchain
• Added a new script to create a minimal sysroot that can be used by cross-compile toolchains
• Refactors to build scripts to support more generic cross-compile support, making it easier to support new architectures in the future.
• Added a bunch of documentation that describes how to setup a cross-compiler environment.
• Deleted some old scripts related to sysroot setup and updating.

Merge checklist:

• [x] test on Raspberry Pi Zero
• [x] test on Raspberry Pi 3
• [x] test on macOS
• [x] test on x86_64 Linux
• [x] add documentation for standalone building
• [x] add documentation for cross-compiling
• [x] test non-standalone build on macOS
• [x] test non-standalone build on x86_64 Linux
• [x] update CHANGELOG and commit hash