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>

 # Compute AVR32-specific values

 CT_DoArchTupleValues() {

     # The architecture part of the tuple:

     CT_TARGET_ARCH="${CT_ARCH}"

     # gcc ./configure flags

     CT_ARCH_WITH_ARCH=

     CT_ARCH_WITH_ABI=

     CT_ARCH_WITH_CPU=

     CT_ARCH_WITH_TUNE=

     CT_ARCH_WITH_FPU=

     CT_ARCH_WITH_FLOAT=

     CT_TARGET_SYS=none

 }