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 E. MORIN"

 reporter_url="http://ymorin.is-a-geek.org/projects/crosstool"

 reporter_comment="EXPERIMENTAL MIPS-64 n64 toolchain."