gallium/util: Don't stub u_debug_stack on Android
The fallback path for no libunwind ends up being stubs for Android. Don't compile them in so we can provide our own implementation. Signed-off-by:Stefan Schake <stschake@gmail.com> Reviewed-by:
Eric Engestrom <eric.engestrom@imgtec.com> Reviewed-by:
Tapani Pälli <tapani.palli@intel.com>