[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
3 files changed