[OpenMP] libomptarget: Disable on MacOS X

Disable compilation of libomptarget on MacOS X.

Differential Revision: https://reviews.llvm.org/D31055



git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@298411 91177308-0d34-0410-b5e6-96231b3b80d8