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.

# powerpc specific configuration file

config ARCH_powerpc

    select ARCH_USE_MMU

    select ARCH_SUPPORT_ABI

    select ARCH_SUPPORT_CPU

    select ARCH_SUPPORT_TUNE

    help

      The PowerPC architecture, as defined by:

        http://www.ibm.com/developerworks/eserver/articles/archguide.html