[RELAY] Avoid eager creation of global target object

[tqchen]

https://github.com/dmlc/tvm/blob/master/src/relay/backend/build_module.cc#L54

Not all target is enabled.

• See https://discuss.tvm.ai/t/unknown-target-name-vpi/2589/3
• Consider changing the map to a function, with switch case

It is usually not a good practice to eagerly create a global static object(which might have initialization order issue).

[tqchen]

cc @zhiics please fix ASAP

cc @jroesch @vinx13 @wweic @hlu1

[zhiics]

I will move them back to std::string and create when necessary.

[tqchen]

#3198

[zhiics]

@tqchen Thanks for the fix.