Makefile support requested - enabling multilib for target

[NightStrike <nightstrike at gmail dot com>]

Currently, gcc doesn't support a multilib build for win64.  I have
been looking at how to do this, and have so far come up with a
beginning to a solution.  The work done thus far is part of this PR:

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38294

The current blocker is in building libgcc.  At some point in building
libgcc, it gets down to the final compilation.  In doing so, gcc is
invoked with a -B option pointing to the 32-bit lib directory instead
of the 64-bit lib directory.  This works for building the 32-bit
libgcc, but not the 64-bit default version.

>From what I can tell, the -B option pointing to the 32-bit lib
directory is in $(GCC_FOR_TARGET).  Is that where it's supposed to be?
 Is there a way to make gcc search the right directory first (or at
all)?  What steps am I missing for enabling the multilib build?

For reference, the system root and all of its libraries are installed into:

$prefix/$target/lib and $prefix/$target/lib64, the latter of course
being the 64-bit version of all the libs.