Don't use libunwindbacktrace for non-ARM.

After fixing the link order here, it seems libunwindbacktrace no
longer works for exception unwinding, but libgcc now does.

Change-Id: Ice79af4b8f234ed6a053ee818b6dec3aa9706c8a

core/cxx_stl_setup.mk

@@ -68,8 +68,6 @@ ifneq ($(filter $(my_cxx_stl),libc++ libc++_static),)
     else
         ifeq (arm,$($(my_prefix)$(LOCAL_2ND_ARCH_VAR_PREFIX)ARCH))
             my_static_libraries += libunwind_llvm
-        else
-            my_static_libraries += libunwindbacktrace
         endif
 
         ifeq ($(my_link_type),static)