WIP: Add clean-base/linux-arm64
This adds a clean-base build script for arm64.
It is not particularly elegant but it does what it needs to do up to cpm, which surfaces a bug in the code generator.
It uses the x64 bootstrap version for 64-bit ABC code of the compiler (but does not use any make scripts from the bootstrap version because they are for x64). Ideally in the future either a arm64 bootstrap version is made available or the bootstrap version is stripped down so that it does not contain make scripts specific to any architecture. Then it also makes sense to move downloading of the bootstrap version to setup.sh (as is the case in linux-x64).
Anyone is free to continue with this when needed.