Implement __aeabi_f2d and __aeabi_d2f

Note1: those are using the correct calling convention, they will read the argument from r0(+r1) because (all?) __aeabi_ functions should use softfp.

Note2: math.h breaks build (is it needed?):
```
math.c:67:7: error: expected ')' before '/' token
```
Note3: I'm not sure if I'm supposed to handle overflows or special values, hopefully someone who knows better would improve this.