libc/newlib: add option to compile support for float IOs

Adds support to enable/disable IOs of floating point values
(float, double, and long double).

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 reporter_name="Ioannis E. VENETIS"

 reporter_url="http://sourceware.org/ml/crossgcc/2009-11/msg00109.html"

 reporter_comment=""