[CMake] Link to compiler-rt if LIBUNWIND_USE_COMPILER_RT is ON.

Summary:
If `-nodefaultlibs` is given, we weren't actually linking to it. This
was true irrespective of passing `-rtlib=compiler-rt` (see previous
patch). Now we explicitly link it to handle that case.

I wonder if we should be linking these libraries only if we're using
`-nodefaultlibs`...

Reviewers: beanz

Subscribers: dberris, mgorny, christof, chrib, cfe-commits

Differential Revision: https://reviews.llvm.org/D51657

llvm-svn: 343990