HIPRTC support for NanoVDB

[meistdan]

I was able to compile NanoVDB using HIPRTC with the following minor changes:

• We need to use __HIPCC_RTC__ to switch to runtime path in util/Util.h
• HIPRTC does not support std::initializer_list

[linux-foundation-easycla]

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[danrbailey]

This PR does not have a signed CLA and the commits have not been signed off so the DCO check is failing. See the contributing document for more details, thank you.

https://github.com/AcademySoftwareFoundation/openvdb/blob/master/CONTRIBUTING.md