When building multilib libraries, CC/CXX etc are set with an option
-B*/lib/, instead of -B<something>/lib/<multi-os-directory>.
This will make some trouble in some case, for example building
cross toolchain based on Debian's cross packages:

  If we have libc6-dev-i386-amd64-cross packages installed on
  a non-x86 machine. This package will have the files in
  /usr/x86_4-linux-gnu/lib32.  The fellow configure will fail
  when build libgcc for i386, with complains the libc is not
  i386 ones:
     ../configure --enable-multilib --enable-multilib \
            --target=x86_64-linux-gnu

Let's insert a "-B*/lib/`CC ${flags} --print-multi-os-directory`"
before "-B*/lib/".

This patch is based on the patch used by Debian now.

ChangeLog

	* config-ml.in: Insert an -B option with multi-os-dir into
	compiler commands used to build libraries.