[BugFix]fix exception when tvm not built with llvm support

[ddwolf]

the exception is like this:

  File "/home/rss/duxiutao/repos/tvm/python/tvm/target/codegen.py", line 80, in llvm_lookup_intrinsic_id
    return _ffi_api.llvm_lookup_intrinsic_id(name)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: module 'tvm.target._ffi_api' has no attribute 'llvm_lookup_intrinsic_id'

[cbalint13]

Hi @ddwolf ,

Thanks for the fix, this would enhance the user experience in the case of llvm absence.
Looks good, but wonder if we could not check the very llvm module presence instead.

[cbalint13]

Thanks @ddwolf !