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 Alpha-specific values

CT_DoArchTupleValues () {

    # The architecture part of the tuple:

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

}