libc/uClibc: add option to enable fenv

Support for fenv.h is a little bit more tricky that enabling it only
for x86-32 is not right.

Add an option for the user to choose whther to install fenv.h or not.

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

 reporter_name="Yann Diorcet <diorcet.yann@gmail.com>"

 reporter_url=""

 reporter_comment="Config to build cross-compiler with mingw as kernel type for

 64 bits Windows target"