cc/gcc: add option to enable/disable decimal floats

Decimal floats need support form the C library, namely support
for fenv, which is missing in uClibc for any architecture but
x86/32.

Add an option (a choice) to enable or disable decimal floats.

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

 32 bits Windows target"