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 binutils-2.20.1.orig/ld/ldmain.c binutils-2.20.1/ld/ldmain.c

 --- binutils-2.20.1.orig/ld/ldmain.c	2010-08-17 19:32:13.000000000 +0200

 +++ binutils-2.20.1/ld/ldmain.c	2010-08-17 19:32:15.000000000 +0200

 @@ -262,6 +262,7 @@

    link_info.allow_undefined_version = TRUE;

    link_info.keep_memory = TRUE;

 +  link_info.new_dtags = TRUE;

    link_info.combreloc = TRUE;

    link_info.strip_discarded = TRUE;

    link_info.emit_hash = TRUE;