Fix part of PR11223 and probably a few dups as well. This teaches the

library search logic to "properly" handle multiarch installations. I've
tested this on both Debian unstable and the latest Ubuntu which both use
this setup, and this appears to work largely the same way as GCC does.
It isn't exactly the same, but it is close enough and more principled in
its behavior where it differs. This should resolve any failures to find
'crt1.o' etc on Debian-based Linux distributions. If folks find more
cases where we fail, please file bugs and CC me.

Test cases for all of the debian silliness are waiting both to simplify
the process of merging these down into the 3.0 release, and because
they're so crazy I haven't yet been able to really produce a fake tree
that represents what we need to test for. I'll eventually add them
though.

llvm-svn: 143344