[libomptarget] Handle offload policy in push_tripcount

If the first target region in a program calls the push_tripcount
function, libomptarget didn't handle the offload policy correctly.
This could lead to unexpected error messages as seen in
http://lists.llvm.org/pipermail/openmp-dev/2019-June/002561.html

To solve this, add a check calling IsOffloadDisabled() as all other
entry points already do. If this method returns false, libomptarget
is effectively disabled.

Differential Revision: https://reviews.llvm.org/D64626

llvm-svn: 366810