cc/gcc: build lto-plugin if binutils' gold is built

To properly enable LTO with gold, gcc has to install a plugin that gold
uses to handle the LTO information.

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

# Compute IA-64-specific values

CT_DoArchTupleValues() {

    # The architecture part of the tuple:

    CT_TARGET_ARCH="${CT_ARCH}"

}