complibs: split companion libraries to backend/frontend, a-la cc_core

Move the actual complibs codes to backend functions that builds the
required combo of build/host/target as requested by a frontend.

This split is currently a no-op, but is required for the upcoming
canadian-cross rework, where we'll be needing to build the complibs
twice, one for build/build, and one for build/host.

This applies to the six companion libraries:
- GMP
- MPFR
- PPL
- Cloog/PPL
- MPC
- libelf

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

COPYING