[chibios] Fix FPU calculations for STM32F4xx

* Use softfp ABI instead of hard ABI

See http://doc.ironwoodlabs.com/arm-arm-none-eabi/html/getting-started/sec-armfloat.html for details.

To be able to use hard float ABI, everything would have to be compiled with such an option. It is probably doable, I just don't think it is worth the effort (what is the performance difference between softfp ABI and hard ABI).

So far, just switching from no-FPU to softfpu gave me measurable performance improvement (well, 1% CPU usage w "softfpu"  vs. 2% CPU usage with no-fpu)