Merge cortex-a-rt and cortex-r-rt

[jonathanpallant]

These two libraries are so similar, I think we could support both processors with the same library. This also makes it easier to add ARMv4T, ARMv5TE and ARMv6 support.

I guess I'd call it cortex-ar-rt. Or, we could call the libraries aarch32-rt and aarch32-cpu, to match aarch64-rt and aarch64-cpu.

[jonathanpallant]

See #78

[robamu]

I like aarch32-cpu and aarch32-rt. Might be a good idea to mentioned Cortex-A and Cortex-R both in the README, package keywords, and description. The FPU specific code is different, but I guess we have different conditional code for this then?

[jonathanpallant]

The only FPU code we have is the "FPU Enable" code, and that's the same. The .fpu directive for the assembler is just to allow us to push and pop the FPU registers when entering a handler and I think all Arm FPUs work the same here, so we can just pick the lowest one that works.