debug/gdb: fix $need_expat_src usage for native-gdb

For the native-gdb (ie on the target), we unconditionally
need to build expat.

Make it a backend, it makes a litle bit cleaner code.

Reported-by: Trevor Woerner <twoerner@gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

# Compute sparc-specific values

CT_DoArchTupleValues() {

    # That's the only thing to override

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

        CT_TARGET_ARCH="sparc64${CT_ARCH_SUFFIX}"

    fi

}