complibs: introduce generic multi-complibs infrastructure

Use the same method as companion tools for providing generic and
extendable companion libs.

Signed-off-by: Yann Diorcet <diorcet.yann@gmail.com>
Message-Id: <515c5c4635d99ebe4877.1353074410@macbook-smorlat.local>
Patchwork-Id: 199613

 # Compute s390-specific values

 CT_DoArchTupleValues() {

     # That's the only thing to override

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

         CT_TARGET_ARCH="s390x"

     fi

 }