[Flang-rt] Forward libomp-mod dependency for non-default runtime builds

Summary:
Currently this is required to inject the dependnecy so the object is
built when targeting OpenMP. This was only applied to the
`runtime_register_default_target` which only applies to
`LLVM_ENABLE_RUNTIMES`. The user could concievably do cross-compiling,
or just manually specify the host architecture and it would then error.

Author: jhuber6

llvm/runtimes/CMakeLists.txt

@@ -320,6 +320,10 @@ function(runtime_register_target name)
     set(install-${runtime_name}-${name} install-${runtime_name})
     set(install-${runtime_name}-${name}-stripped install-${runtime_name}-stripped)
     list(APPEND ${name}_extra_targets ${runtime_name}-${name} install-${runtime_name}-${name} install-${runtime_name}-${name}-stripped)
+    # We need the target 'libomp-mod' to run flang-rt OpenMP tests.
+    if(${runtime_name} STREQUAL "openmp")
+      list(APPEND ${name}_extra_targets "libomp-mod")
+    endif()
     if(LLVM_INCLUDE_TESTS)
       set(check-${runtime_name}-${name} check-${runtime_name} )
       list(APPEND ${name}_test_targets check-${runtime_name}-${name})