cxx_stl_setup is not executed for modules from soong

cxx_stl_setup is adding libunwind_llvm to the dependency even for
modules from soong. However, since this dependency was already handled
in soong, those modules don't need to go through it again.

Bug: 64815735
Test: BOARD_VNDK_VERSION=current mma -j (under /hardware/libhardware)

Change-Id: Id6e2547c752ec831aa56a14c0bf4db0a55d51243