cc/gcc: add option to enable 128-bit long doubles

Needed by some PPC targets, at least.
Requires gcc 4.2+ (noticed by Arnaud LACOMBE).

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 reporter_name="Harold Grovesteen"

 reporter_url="http://sourceware.org/ml/crossgcc/2009-11/msg00052.html"

 reporter_comment=""