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 -dur uClibc-0.9.28.1.orig/libc/sysdeps/linux/common/Makefile uClibc-0.9.28.1/libc/sysdeps/linux/common/Makefile

--- uClibc-0.9.28.1.orig/libc/sysdeps/linux/common/Makefile	2007-01-26 00:54:19.000000000 +0100

+++ uClibc-0.9.28.1/libc/sysdeps/linux/common/Makefile	2007-02-01 08:44:37.000000000 +0100

@@ -25,6 +25,10 @@

 SRCS := $(filter-out sbrk.c,$(SRCS))

 endif

+ifneq ($(strip $(UCLIBC_HAS_LFS)),y)

+SRCS := $(filter-out readahead.c,$(SRCS))

+endif

+

 ifneq ($(strip $(UCLIBC_HAS_SSP)),y)

 SRCS := $(filter-out ssp.c,$(SRCS))

 endif