Enable generic multilib support on 32bit hosts. Previously this was only

enabled for debian hosts, which is quite odd. I think all restriction on
when Clang attempts to use a multilib installation should go away. Clang
is fundamentally a cross compiler. It behaves more like GCC when built
as a cross compiler, and so it should just use multilib installs when
they are present on the system. However, there is a very specific
exemption for Exherbo, which I can't test on, so I'm leaving that in
place.

With this, check in a generic test tree for multilib on a 32-bit host.
This stubs out many directories that most distributions don't use but
that uptsream GCC supports. This is intended to be an agnostic test that
the driver behaves properly compared with the GCC driver it aims for
compatibility with.

Also, fix a bug in the driver that this testing exposed (see!) where it
was incorrectly testing the target architecture rather than the host
architecture.

If anyone is having trouble with the tree-structure stubs I'm creating
to test this, let me know and I can revisit the design. I chose this
over (for example) a tar-ball in order to make tests run faster at the
small, hopefully amortized VCS cost.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@140999 91177308-0d34-0410-b5e6-96231b3b80d8