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>

# windows config options

## depends on ARCH_x86

##

## select WINDOWS

##

## help Build a toolchain targeting systems running Windows as host