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>

 diff -durN ltrace-0.4.orig/Makefile.in ltrace-0.4/Makefile.in

 --- ltrace-0.4.orig/Makefile.in	2008-10-26 15:34:21.000000000 +0100

 +++ ltrace-0.4/Makefile.in	2008-10-26 15:37:40.000000000 +0100

 @@ -5,6 +5,7 @@

  OS		:= @HOST_OS@

  ARCH		:= $(shell echo "@HOST_ARCH@" |sed -e s/i.86/i386/      \

  						   -e s/sun4u/sparc64/  \

 +						   -e s/alpha.*/alpha/  \

  						   -e s/arm.*/arm/      \

  						   -e s/sa110/arm/      \

  						   -e s/ppc64/ppc/      \