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>

 # Compute Alpha-specific values

 CT_DoArchTupleValues () {

     # The architecture part of the tuple:

     CT_TARGET_ARCH="${CT_ARCH}${CT_ARCH_ALPHA_VARIANT}"

 }