toolchain/crosstool-NG: disable decimal floats

Decimal floats were introduced circa gcc-4.2 or -4.3, and requires
the floating-point environement fenv.h in the C library.

The uClibc .config file used by crosstool-NG to build uClibc is the
same as used by the internal buildroot mechanism, and explcitly
disables fenv support.

The quick workaround is to simply disable decimal floats in all
crosstool-NG config files.

In the long run, it might be better to check this situation, and/or
add code and/or options in crosstool-NG to handle this (but it is
much more involved, and this workaround is sane).

Reported-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
CC: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>