There wasn't any particular reason the functions needed to be there
anyway, so just get rid of them, and adjust libstd to compensate.

With this change, libcore depends on exactly two floating-point functions:
fmod and fmodf.  They are implicitly referenced because they are used to
implement "%".

Dependencies of libcore on Linux x86-x64 with this patch:
```
0000000000000000         *UND*	0000000000000000 __powidf2
0000000000000000         *UND*	0000000000000000 __powisf2
0000000000000000         *UND*	0000000000000000 fmod
0000000000000000         *UND*	0000000000000000 fmodf
0000000000000000         *UND*	0000000000000000 memcmp
0000000000000000         *UND*	0000000000000000 memcpy
0000000000000000         *UND*	0000000000000000 memset
0000000000000000         *UND*	0000000000000000 rust_begin_unwind
0000000000000000         *UND*	0000000000000000 rust_eh_personality
```