Build compiler-rt against libunwind, not libcxxrt

Parts of compiler-rt are also built for libgcc_eh and libgcc_s, and
these were already pointing to the libunwind unwind.h. For the sake of
consistency, also build compiler-rt itself against the libunwind
unwind.h, not the libcxxrt one.

MFC after: 3 days

(cherry picked from commit 7ecd99fa424df001028c5cddc52d25b29232f1af)