[OpenMP] Only include CMAKE_DL_LIBS on unix platforms

[mstorsjo]

Summary

CMAKE_DL_LIBS is documented as "Name of library containing dlopen
and dlclose".

On Windows platforms, there's no system provided dlopen/dlclose, but
it can be argued that if you really intend to call dlopen/dlclose,
you're going to be using a third party compat library like
https://github.com/dlfcn-win32/dlfcn-win32, and CMAKE_DL_LIBS should
expand to its name.

This has been argued upstream in CMake in
https://gitlab.kitware.com/cmake/cmake/-/issues/17600 and
https://gitlab.kitware.com/cmake/cmake/-/merge_requests/1642, that
CMAKE_DL_LIBS should expand to "dl" on mingw platforms.

The merge request wasn't merged though, as it caused some amount of
breakage, but in practice, Fedora still carries a custom CMake patch
with the same effect.

Thus, this patch fixes cross compiling OpenMP for mingw targets
on Fedora with their cutom-patched CMake.

[JonChesterfield]

OK. It's a bit of an edge case but the patch is reasonable. In general the hope is to move to using llvm libs to abstract over things like dlsym but it's a gradual process.

openmp/runtime/src/CMakeLists.txt

 # We neither need (nor want) the standard C++ library dependency even though we compile c++ files.
 if(NOT ${LIBOMP_USE_STDCPPLIB})
   set(LIBOMP_LINKER_LANGUAGE C)
   set(CMAKE_CXX_IMPLICIT_LINK_LIBRARIES)
 else()
   set(LIBOMP_LINKER_LANGUAGE CXX)
 endif()
 
+if(UNIX)
+  set(LIBOMP_DL_LIBS ${CMAKE_DL_LIBS})
+endif()
+
 # Add the OpenMP library
 libomp_get_ldflags(LIBOMP_CONFIGURED_LDFLAGS)
 
 libomp_get_libflags(LIBOMP_CONFIGURED_LIBFLAGS)
 # Build libomp library. Add LLVMSupport dependency if building in-tree with libomptarget profiling enabled.
 if(OPENMP_STANDALONE_BUILD OR (NOT OPENMP_ENABLE_LIBOMP_PROFILING))
   add_library(omp ${LIBOMP_LIBRARY_KIND} ${LIBOMP_SOURCE_FILES})
   # Linking command will include libraries in LIBOMP_CONFIGURED_LIBFLAGS
-  target_link_libraries(omp ${LIBOMP_CONFIGURED_LIBFLAGS} ${CMAKE_DL_LIBS})
+  target_link_libraries(omp ${LIBOMP_CONFIGURED_LIBFLAGS} ${LIBOMP_DL_LIBS})
 else()
   add_llvm_library(omp ${LIBOMP_LIBRARY_KIND} ${LIBOMP_SOURCE_FILES} PARTIAL_SOURCES_INTENDED
-    LINK_LIBS ${LIBOMP_CONFIGURED_LIBFLAGS} ${CMAKE_DL_LIBS}
+    LINK_LIBS ${LIBOMP_CONFIGURED_LIBFLAGS} ${LIBOMP_DL_LIBS}
     LINK_COMPONENTS Support
     )
   # libomp must be a C++ library such that it can link libLLVMSupport
   set(LIBOMP_LINKER_LANGUAGE CXX)
 endif()
 if(${LIBOMP_USE_HWLOC})
   target_include_directories(omp
                              PUBLIC