skip gcc.target/arm/div64-unwinding.c on vxworks_kernel targets

skip gcc.target/arm/div64-unwinding.c on vxworks_kernel targets

This test verifies, by using a weak reference to _Unwind_RaiseException,
that performing division by zero does not cause that symbol to get
indirectly pulled into our closure.

The testing methodology unfortunately does not work on VxWorks targets
when building in kernel mode. This is inherent to how kernel mode
on VxWorks works: The link is only partial and the remaining symbols
which have not been resolved already get automatically resolved by
the VxWorks loader at the moment the module is loaded onto the target,
prior to execution. The resolution includes weak symbols too, which
defeats the purpose of this test.

gcc/testsuite/

        * gcc.target/arm/div64-unwinding.c: Skip on vxworks_kernel targets.