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>

# Alpha specific configuration file

## select ARCH_SUPPORTS_32

## select ARCH_DEFAULT_32

## select ARCH_USE_MMU

## select ARCH_SUPPORTS_WITH_CPU

## select ARCH_SUPPORTS_WITH_TUNE

##

## help The Alpha architecture.