[ARM] implement fibers & Add internal Fiber documentation

This implements Fibers on ARM. Tested on a ARMv5 ```SoftFloat``` system without FPU and on the ARMv6 Raspberry with FPU and ```HardFloat``` ABI & GDC. druntime unit tests and the vibe-d examples are working.

I also added some internal documentation which should make porting Fibers to new architectures easier for people who never did that before.