libcxx: Fix ARM libc++/abi and libunwind buildbot.

The test cases were crashing due to the mixed usage of the unwinding
functions from both libunwind and libgcc_s.  The unwind functions are
mixed because the "llvm_unwinder" entry is not available in the
lit.site.cfg for libc++.  As a result, "-lgcc_s" is picked instead of
"-lunwind".  The extra option to lit --param=link_flags="-lunwind" won't
help either.

This CL fix the problem by adding llvm_unwinder to lit.site.cfg.in.

llvm-svn: 237518