Use old unwinder for R cc modules

Modules linking against VNDK R have been using libgcc_stripped and
libunwind_llvm as their unwinder. This change uses those old unwinder
libraries if current module's VNDK version is R.

Bug: 178470649
Test: soong test
Change-Id: I86d0600b2fb09cd9828c0a5b1157558711b0ec86