Properly set LOCAL_CXX_STL where needed

This sets _USING_LIBCXX correctly, causing <stdatomic.h> to correctly detect
that <atomic> is available, preventing compilation failures when both are
included.

Removed prior manual approximations that manually set other variables
that are now set automatically.

Change-Id: Ia0297404af08d7d3b42679d402a4a8048b5f05d5
1 file changed