binutils: split binutils to backend/frontend, a-la cc_core

Move the actual binutils code to a backend function 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 two binutils,
one for build/build/target, and one for build/host/target.

This applies to the three binutils:
- GNU binutils
- elf2flt
- sstrip

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

 diff -durN glibc-2.7.orig/Makerules glibc-2.7/Makerules

 --- glibc-2.7.orig/Makerules	2007-08-26 04:18:03.000000000 +0200

 +++ glibc-2.7/Makerules	2008-07-28 15:17:27.000000000 +0200

 @@ -1203,6 +1203,9 @@

  # The include magic above causes those files to use this variable for flags.

  CPPFLAGS-nonlib = -DNOT_IN_libc=1

 +ifneq ($(OBJDUMP_FOR_HOST),)

 +OBJDUMP = $(OBJDUMP_FOR_HOST)

 +endif

  ifeq ($(versioning),yes)

  # Generate normalized lists of symbols, versions, and data sizes.