Support Debian s390x multiarch paths

Summary: Patch by Steven Chamberlain <steven@pyro.eu.org>

Reviewers: uweigand

Differential Revision: http://reviews.llvm.org/D12430

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@246295 91177308-0d34-0410-b5e6-96231b3b80d8