test-suite: apply cleanup pass

Remove all non-modifiable items (target tuple, gcc version, toolchain path...)
Makefile syntax ( use $(...) instead of ${...} )
Update doc
Space-damage cleanups

# Compute s390-specific values

CT_DoArchTupleValues() {

    # That's the only thing to override

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

        CT_TARGET_ARCH="s390x"

    fi

}