[libcxxabi] Add -funwind-tables to the test compilation options.

Summary:
backtrace_test.pass.cpp depends on unwind tables. These are generated by
-funwind-tables which is the default for x86 but not for other targets.

Thanks to Nitesh Jain for helping to narrow this down.

Fixes PR24148

Reviewers: jroelofs

Subscribers: cfe-commits, jroelofs, llvm-commits, hans

Differential Revision: http://reviews.llvm.org/D11529

llvm-svn: 243296