remove math code for newlib

Newlib already have optimized and full featured math implementation. To
use it, one should add:

    env['LIBS'] = ['m']

or equivalent to the SConstruct.
Newlib: link with libm in default.

libm is a frequently used lib. Newlib is compiled with
-ffunction-sections in recent GCC tool chains. The linker would just
link in the functions that have been referenced. So setting this won't
result in bigger text size.
newlib: use the LIBS argument in DefineGroup instead of modifying the Env
Merge pull request #11 from grissiom/remove-newlib-math

remove math code for newlib