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>

 --- gcc-4.1.2.orig/libstdc++-v3/configure	2007-03-04 23:32:31.000000000 +0100

 +++ gcc-4.1.2/libstdc++-v3/configure	2007-03-04 23:33:54.000000000 +0100

 @@ -7324,6 +7324,9 @@

  cat >>conftest.$ac_ext <<_ACEOF

  /* end confdefs.h.  */

  #include <complex.h>

 +#ifdef __UCLIBC__

 +#error ugly hack to make sure configure test fails here for cross until uClibc supports the complex funcs

 +#endif

  int

  main ()

  {