Move cxx_stl_setup.mk out of ifdef

This commit moves `include cxx_stl_setup.mk` out of `ifdef
my_shared_libraries` so that it can always get a chance to tweak
my_shared_libraries.

Bug: 137302468
Test: lunch aosp_walleye-userdebug && make
Test: lunch aosp_walleye-userdebug && make check-elf-files
Change-Id: Id756340f0a62d7ac06d301c3260430a1af74ed89