[v3] Use GCC's math builtins in std_cmath.h

[Loren James Rittle]

> It then turns out that doing this, std_cmath no longer needs to provide
> function prototypes or using directives for the C library's versions in
> the __gnu_cxx::__c99_binding namespace.  I'm not sure if this adversely
> affects the ABI, but the patch below simply removes these statements
> now that they're no longer needed. [...]

> Ok for mainline??

Roger, as the author of the hack you just removed: Sweet!

Regards,
Loren