This script are modified versions of the original avr-toolchain script used by Arduino. I modified it to cache downloads and extracted archives for reduced build times. I also added command support as good as I could. But everything is still extremely crude.
Disclaimer: This script is not really reliable. Since I do not know much of Bash, please feel free to suggest corrections when you see face-palming code.
I was often (un-) commenting some lines when building so you might have to play around a bit.
Things were buildable in a Ubuntu x64 Virtual Machine for 64-bit Windows and Linux. 32-bit Windows always failed because of redefinitions.
Before you can Cross-Compile, e.g. compiling a windows binary in linux, you have to compile the build system binaries first as gcc has to compile the start-up files. The objdir can than be simply renamed to "avr-gcc-buildsys" or whatever you define in the config file.
You will also have to specify a download folder for the archives. The pre-defined folder-name is "downloads". As github does not make you upload empty folders, you need to create one yourself.