Fix more fallout from multilib shared libgcc rearrangement

[Richard Sandiford]

Zack Weinberg <zack@codesourcery.com> writes:
> This, I hope, is the last bit of fallout from all the libgcc build
> changes.  As David Edelsohn discovered a couple weeks ago, it is now
> necessary for SHLIB_INSTALL to create the directory it is installing
> into, because that directory doesn't get created earlier.

Thanks, and sorry to everyone for all the breakage.