[RUNTIME] Correctly handling export_module when exporting modules of different type

[jinhongyii]

Previously the export_module function assumes that the module and all imported modules have the same type_key. If we have a host llvm module and some imported c modules, it may fail to set the llvm_target_string and object_format correctly, and thus fail to continue with the llvm build flow.

In this PR, I set the object format of a mixed-type module to that of the host module, and always set llvm_target_string when we see a host llvm module.

cc: @tqchen @YuchenJin

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• cc @areusch _{See #10317 for details}
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[tqchen]

Thanks hongyi. This is a hidden issue as previous behavior will still go through the c export route, and not the llvm one. Which is still correct for small module size. But it will cost long export time for bigger models. cc @jwfromm