tools wrapper: introduce the silent WRAPPER_NEEDED config option

Add the WRAPPER_NEEDED silent config option, that can be selected by
components that require it (companion libs so far).
Rely on this config option when deciding to install the wrapper,
instead of checking GMP/MPFR or PPL/CLoog/MPC.

# Compute Alpha-specific values

CT_DoArchTupleValues () {

    # The architecture part of the tuple:

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

    # The kernel ARCH:

    CT_KERNEL_ARCH=${CT_ARCH}

}