cc/gcc: Make usage of --enable-target-optspace configurable

Optionally configure to compile gcc-internal libs with -Os.

yann.morin.1998@anciens.enib.fr:
Rename the config options, it is in the GCC /namespace/.

 # Compute s390-specific values

 CT_DoArchTupleValues() {

     # That's the only thing to override

     if [ "${CT_ARCH_64}" = "y" ]; then

         CT_TARGET_ARCH="s390x"

     fi

 }