Quicksilver OS Kit is for linux. It has a script to install a gcc cross compiler, into any architecture. It also includes some libraries to start an os with.
Prerequisites: GCC cross compiler, linux
The default architecture for OS Kit NEXT is x86_64-elf. To change this, in the first line in build.sh change x86_64-elf to:i686-elf, aarch64-elf, or arm-eabi.
The builder will come out sometime later.