[OpenMP][libomp] Implement KMP_DLSYM_NEXT on Windows The interop API routines try to invoke external entries, but we did not have support for KMP_DLSYM_NEXT on Windows. Also added proper guards for STUB build. Differential Revision: https://reviews.llvm.org/D149892 GitOrigin-RevId: 2f6eba7516b267bf3bbfee840fd61f7fb8cde1c3